Implement a basic TLS 1.3 client handshake, only enabled if explicitly
requested with MaxVersion.

This CL intentionally leaves for future CLs:
  - PSK modes and resumption
  - client authentication
  - post-handshake messages
  - downgrade protection
  - KeyLogWriter support

Updates #9671

Change-Id: Ieb6130fb6f25aea4f0d39e3a2448dfc942e1de7a
Reviewed-on: https://go-review.googlesource.com/c/146559
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

crypto/x509 already supports PSS signatures (with rsaEncryption OID),
and crypto/tls support was added in CL 79736. Advertise support for the
algorithms and accept them as a peer.

Note that this is about PSS signatures from regular RSA public keys.
RSA-PSS only public keys (with RSASSA-PSS OID) are supported in neither
crypto/tls nor crypto/x509. See RFC 8446, Section 4.2.3.

testdata/Server-TLSv12-ClientAuthRequested* got modified because the
CertificateRequest carries the supported signature algorithms.

The net/smtp tests changed because 512 bits keys are too small for PSS.

Based on Peter Wu's CL 79738, who did all the actual work in CL 79736.

Updates #9671

Change-Id: I4a31e9c6e152ff4c50a5c8a274edd610d5fff231
Reviewed-on: https://go-review.googlesource.com/c/146258
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

RFC 8446 recommends using the supported_versions extension to negotiate
lower versions as well, so begin by implementing it to negotiate the
currently supported versions.

Note that pickTLSVersion was incorrectly negotiating the ServerHello
version down on the client. If the server had illegally sent a version
higher than the ClientHello version, the client would have just
downgraded it, hopefully failing later in the handshake.

In TestGetConfigForClient, we were hitting the record version check
because the server would select TLS 1.1, the handshake would fail on the
client which required TLS 1.2, which would then send a TLS 1.0 record
header on its fatal alert (not having negotiated a version), while the
server would expect a TLS 1.1 header at that point. Now, the client gets
to communicate the minimum version through the extension and the
handshake fails on the server.

Updates #9671

Change-Id: Ie33c7124c0c769f62e10baad51cbed745c424e5b
Reviewed-on: https://go-review.googlesource.com/c/146217
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

Note that there is significant code duplication due to extensions with
the same format appearing in different messages in TLS 1.3. This will be
cleaned up in a future refactor once CL 145317 is merged.

Enforcing the presence/absence of each extension in each message is left
to the upper layer, based on both protocol version and extensions
advertised in CH and CR. Duplicated extensions and unknown extensions in
SH, EE, HRR, and CT will be tightened up in a future CL.

The TLS 1.2 CertificateStatus message was restricted to accepting only
type OCSP as any other type (none of which are specified so far) would
have to be negotiated.

Updates #9671

Change-Id: I7c42394c5cc0af01faa84b9b9f25fdc6e7cfbb9e
Reviewed-on: https://go-review.googlesource.com/c/145477Reviewed-by: Adam Langley <agl@golang.org>

Vendors golang.org/x/crypto/hkdf at e84da0312774c21d64ee2317962ef669b27ffb41

Updates #9671

Change-Id: I2610c4a66756e2a6f21f9823dcbe39edd9c9ea21
Reviewed-on: https://go-review.googlesource.com/c/145298Reviewed-by: Adam Langley <agl@golang.org>

Updates #9671

Change-Id: I1ea7b724975c0841d01f4536eebb23956b30d5ea
Reviewed-on: https://go-review.googlesource.com/c/145297Reviewed-by: Adam Langley <agl@golang.org>

Updates #9671

Change-Id: Ia68224aca866dc3c98af1fccbe56bfb3f22da9f6
Reviewed-on: https://go-review.googlesource.com/c/144578
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

Change-Id: I82e3f9140e5d14f02beef64c474a3ae88fe256e1
Reviewed-on: https://go-review.googlesource.com/c/147219
Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Unlikely to happen in practice, but easy enough to prevent and might
as well do so for completeness.

Fixes #28243.

Change-Id: I848c3af49cb923f088e9490c6a79373e182fad08
Reviewed-on: https://go-review.googlesource.com/c/142719
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Austin Clements <austin@google.com>

Change-Id: I11adca150ab795607b832fb354a3e065655e1020
Reviewed-on: https://go-review.googlesource.com/c/147179Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Change-Id: Iaff03911f1807d462f1966590626bd486807f53d
Reviewed-on: https://go-review.googlesource.com/c/147178Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Fixes #28545

Change-Id: I31c57ce11aca651cacc72235c7753e0c0fd170ef
Reviewed-on: https://go-review.googlesource.com/c/146900
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Fixes #28549

Change-Id: Iba71bb2edd0759004e0c7df92b2b8f1197bd62d3
Reviewed-on: https://go-review.googlesource.com/c/146901
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

There are some OpenGL functions that take more than 15 arguments.
This CL adds Syscall18 to enable to call such functions on Windows
via syscall functions.

Fixes #28434

Change-Id: Ic7e37dda9cadf4516183e98166bfc52844ad2bbe
Reviewed-on: https://go-review.googlesource.com/c/147117Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

Add simple description to String method of IsolationLevel struct.

Change-Id: I8bdf829c81d4895b8542a3f21437bed61c6e925d
GitHub-Last-Rev: 7e16d6a93a9560cf3a09413dfc47d0cb04bb2fde
GitHub-Pull-Request: golang/go#28560
Reviewed-on: https://go-review.googlesource.com/c/147040Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Fixes #28557

Change-Id: Ifca958b78e8c62fbc66515e693f528d799e8e84b
Reviewed-on: https://go-review.googlesource.com/c/147039Reviewed-by: Ian Lance Taylor <iant@golang.org>

This commit skips tests which aren't yet supported on AIX.

nosplit.go is disabled because stackGuardMultiplier is increased for
syscalls.

Change-Id: Ib5ff9a4539c7646bcb6caee159f105ff8a160ad7
Reviewed-on: https://go-review.googlesource.com/c/146939Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

Updates #26794

Change-Id: Ic1d3078176721f3d2e5d8188c234383037babbaf
Reviewed-on: https://go-review.googlesource.com/c/145177
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

This commit fixes tests which fail on some versions of AIX 7.2 due
to internal bugs.

getsockname isn't working properly with unix networks.

Timezone files aren't returning a correct output.

Change-Id: I4ff15683912be62ab86dfbeeb63b73513404d086
Reviewed-on: https://go-review.googlesource.com/c/146940
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

This commit changes poll.PollDescriptor by poll.IsPollDescriptor. This
is needed for OS like AIX which have more than one FD using inside their
netpoll implementation.

Change-Id: I49e12a8d74045c501e19fdd8527cf166a3c64850
Reviewed-on: https://go-review.googlesource.com/c/146938
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

We only need the memory barrier from these stores,
and we only store nil over nil or over a static function value.
The write barrier is unnecessary.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.0µs ± 0%  -0.43%  (p=0.032 n=5+5)
SetReadDeadline-6      205ns ± 1%   205ns ± 1%    ~     (p=0.683 n=5+5)

Update #25729

Change-Id: I66c097a1db7188697ddfc381f31acec053dfed2c
Reviewed-on: https://go-review.googlesource.com/c/146345
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Follow CL 146020 and enable RemoveAll based on Unlinkat and Openat on
aix.

Updates #27029

Change-Id: I78b34ed671166ee6fa651d5f2025b88548ee6c68
Reviewed-on: https://go-review.googlesource.com/c/146937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

runtimeNano is slower than nanotime, so pass the duration
to runtime_pollSetDeadline as is. netpoll can add nanotime itself.
Arguably a bit simpler because, say, a negative duration
clearly represents already expired timer, no need to compare to
nanotime again.
This may also fix an obscure corner case when a deadline in past
which happens to be nanotime 0 is confused with no deadline at all,
which are radically different things.
Also don't compute any durations and times if Time is zero
(currently we first compute everything and then reset d back to 0,
which is wasteful).

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.1µs ± 0%  17.0µs ± 0%     ~     (p=0.421 n=5+5)
SetReadDeadline-6      230ns ± 0%   205ns ± 1%  -10.63%  (p=0.008 n=5+5)

Change-Id: I2aad699270289a5b9ead68f5e44ec4ec6d96baa0
Reviewed-on: https://go-review.googlesource.com/c/146344Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>

We only need the memory barrier in poll_runtime_pollSetDeadline only
when one of the timers has fired, which is not the expected case.
Memory barrier can be somewhat expensive on some archs,
so execute it only if one of the timers has in fact fired.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 0%  +0.35%  (p=0.032 n=5+5)
SetReadDeadline-6      232ns ± 0%   230ns ± 0%  -1.03%  (p=0.000 n=4+5)

Update #25729

Change-Id: Ifce6f505b9e7ba3717bad8f454077a2e94ea6e75
Reviewed-on: https://go-review.googlesource.com/c/146343Reviewed-by: Ian Lance Taylor <iant@golang.org>

The nanotime wrappers in runtime introduce a bunch
of unnecessary code onto hot paths, e.g.:

0000000000449d70 <time.runtimeNano>:
  449d70:       64 48 8b 0c 25 f8 ff    mov    %fs:0xfffffffffffffff8,%rcx
  449d77:       ff ff
  449d79:       48 3b 61 10             cmp    0x10(%rcx),%rsp
  449d7d:       76 26                   jbe    449da5 <time.runtimeNano+0x35>
  449d7f:       48 83 ec 10             sub    $0x10,%rsp
  449d83:       48 89 6c 24 08          mov    %rbp,0x8(%rsp)
  449d88:       48 8d 6c 24 08          lea    0x8(%rsp),%rbp
  449d8d:       e8 ae 18 01 00          callq  45b640 <runtime.nanotime>
  449d92:       48 8b 04 24             mov    (%rsp),%rax
  449d96:       48 89 44 24 18          mov    %rax,0x18(%rsp)
  449d9b:       48 8b 6c 24 08          mov    0x8(%rsp),%rbp
  449da0:       48 83 c4 10             add    $0x10,%rsp
  449da4:       c3                      retq
  449da5:       e8 56 e0 00 00          callq  457e00 <runtime.morestack_noctxt>
  449daa:       eb c4                   jmp    449d70 <time.runtimeNano>

Move them to the corresponding packages which eliminates all of this.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.1µs ± 1%  17.0µs ± 0%  -0.66%  (p=0.032 n=5+5)
SetReadDeadline-6      234ns ± 1%   232ns ± 0%  -0.77%  (p=0.016 n=5+4)

Update #25729

Change-Id: Iee05027adcdc289ba895c5f5a37f154e451bc862
Reviewed-on: https://go-review.googlesource.com/c/146342
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

time.now is somewhat expensive (much more expensive than nanotime),
in the common case when Time has monotonic time we don't actually
need to call time.now in Since/Until as we can do calculation
based purely on monotonic times.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 1%     ~     (p=0.151 n=5+5)
SetReadDeadline-6      261ns ± 0%   234ns ± 1%  -10.35%  (p=0.008 n=5+5)

Benchmark that only calls Until:

benchmark            old ns/op     new ns/op     delta
BenchmarkUntil       54.0          29.5          -45.37%

Update #25729

Change-Id: I5ac5af3eb1fe9f583cf79299f10b84501b1a0d7d
Reviewed-on: https://go-review.googlesource.com/c/146341
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Move startNano from runtime to time package.
In preparation for a subsequent change that speeds up Since and Until.
This also makes code simpler as we have less assembly as the result,
monotonic time handling is better localized in time package.
This changes values returned from nanotime on windows
(it does not account for startNano anymore), current comments state
that it's important, but it's unclear how it can be important
since no other OS does this.

Update #25729

Change-Id: I2275d57b7b5ed8fd0d53eb0f19d55a86136cc555
Reviewed-on: https://go-review.googlesource.com/c/146340Reviewed-by: Ian Lance Taylor <iant@golang.org>

Currently when netpoll deadline is incrementally prolonged,
we delete and re-add timer each time.
Add modtimer function that does both and use it when we need
to modify an existing netpoll timer to avoid unnecessary lock/unlock.

TCP4OneShotTimeout-6  17.2µs ± 0%  17.0µs ± 0%  -0.82%  (p=0.008 n=5+5)
SetReadDeadline-6      274ns ± 2%   261ns ± 0%  -4.89%  (p=0.008 n=5+5)

Update #25729

Change-Id: I08b89dbbc1785dd180e967a37b0aa23b0c4613a8
Reviewed-on: https://go-review.googlesource.com/c/146339Reviewed-by: Ian Lance Taylor <iant@golang.org>

Currently we always delete both read and write timers and then
add them again. However, if user setups read and write deadline
separately, then we don't need to touch the other one.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.2µs ± 0%  17.2µs ± 0%     ~     (p=0.310 n=5+5)
SetReadDeadline-6      319ns ± 1%   274ns ± 2%  -13.94%  (p=0.008 n=5+5)

Update #25729

Change-Id: I4c869c3083521de6d0cd6ca99a7609d4dd84b4e4
Reviewed-on: https://go-review.googlesource.com/c/146338Reviewed-by: Ian Lance Taylor <iant@golang.org>

It's not always necessary to wake timerproc even if we add
a new timer to the top of the heap. Since we don't wake and
reset timerproc when we remove timers, it still can be sleeping
with shorter timeout. It such case it's more profitable to let it
sleep and then update timeout when it wakes on its own rather than
proactively wake it, let it update timeout and go to sleep again.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  18.6µs ± 1%  17.2µs ± 0%   -7.66%  (p=0.008 n=5+5)
SetReadDeadline-6      562ns ± 5%   319ns ± 1%  -43.27%  (p=0.008 n=5+5)

Update #25729

Change-Id: Iec8eacb8563dbc574a82358b3bac7ac479c16826
Reviewed-on: https://go-review.googlesource.com/c/146337Reviewed-by: Ian Lance Taylor <iant@golang.org>

This change uses library functions such as bits.RotateLeft32 to
reduce the amount of code needed in the generic implementation.
Since the code is now shorter I've also removed the option to
generate a non-unrolled version of the code.

I've also tried to remove bounds checks where possible to make
the new version performant, however that is not the primary goal
of this change since most architectures have assembly
implementations already.

Assembly performance:

name                 old speed      new speed      delta
Hash8Bytes           50.3MB/s ± 1%  59.1MB/s ± 0%  +17.63%  (p=0.000 n=9+8)
Hash1K                590MB/s ± 0%   597MB/s ± 0%   +1.25%  (p=0.000 n=9+9)
Hash8K                636MB/s ± 1%   638MB/s ± 1%     ~     (p=0.072 n=10+10)
Hash8BytesUnaligned  50.5MB/s ± 0%  59.1MB/s ± 1%  +17.09%  (p=0.000 n=10+10)
Hash1KUnaligned       589MB/s ± 1%   596MB/s ± 1%   +1.23%  (p=0.000 n=9+10)
Hash8KUnaligned       638MB/s ± 1%   640MB/s ± 0%   +0.35%  (p=0.002 n=10+10)

Pure Go performance:

name                 old speed      new speed      delta
Hash8Bytes           30.3MB/s ± 1%  42.8MB/s ± 0%  +41.20%  (p=0.000 n=9+9)
Hash1K                364MB/s ± 4%   394MB/s ± 1%   +8.27%  (p=0.000 n=10+10)
Hash8K                404MB/s ± 1%   420MB/s ± 0%   +4.17%  (p=0.000 n=10+9)
Hash8BytesUnaligned  30.3MB/s ± 1%  42.8MB/s ± 1%  +40.92%  (p=0.000 n=9+10)
Hash1KUnaligned       368MB/s ± 0%   394MB/s ± 0%   +7.07%  (p=0.000 n=9+9)
Hash8KUnaligned       404MB/s ± 1%   411MB/s ± 3%   +1.91%  (p=0.026 n=9+10)

Change-Id: I9a91fb52ea8d62964d5351bdf121e9fbc9282852
Reviewed-on: https://go-review.googlesource.com/c/137355
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Stat uses Windows FindFirstFile + CreateFile to gather symlink
information - FindFirstFile determines if file is a symlink,
and then CreateFile follows symlink to capture target details.

Lstat only uses FindFirstFile.

This CL replaces current approach with just a call to CreateFile.
Lstat uses FILE_FLAG_OPEN_REPARSE_POINT flag, that instructs
CreateFile not to follow symlink. Other than that both Stat and
Lstat look the same now. New code is simpler.

CreateFile + GetFileInformationByHandle (unlike FindFirstFile)
does not report reparse tag of a file. I tried to ignore reparse
tag altogether. And it works for symlinks and mount points.
Unfortunately (see https://github.com/moby/moby/issues/37026),
files on deduped disk volumes are reported with
FILE_ATTRIBUTE_REPARSE_POINT attribute set and reparse tag set
to IO_REPARSE_TAG_DEDUP. So, if we ignore reparse tag, Lstat
interprets deduped volume files as symlinks. That is incorrect.
So I had to add GetFileInformationByHandleEx call to gather
reparse tag after calling CreateFile and GetFileInformationByHandle.

Fixes #27225
Fixes #27515

Change-Id: If60233bcf18836c147597cc17450d82f3f88c623
Reviewed-on: https://go-review.googlesource.com/c/143578
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>

This CL changes IsAbs to return true for "NUL" and other Windows
reserved filenames (search
https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file
for NUL for details). os.Open("NUL") and os.Stat("NUL") work
regardless of what current directory is, and it is mistake to join
"NUL" with current directory when building full path. Changing
IsAbs("NUL") to return true fixes that mistake.

Fixes #28035

Change-Id: Ife8f8aee48400702613ede8fc6834fd43e6e0f03
Reviewed-on: https://go-review.googlesource.com/c/145220
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Updating each call in place broke when there were multiple cgo calls
used as arguments to another cgo call where some required rewriting.
Instead, rewrite calls to strings via the existing mangling mechanism,
and only substitute the top level call in place.

Fixes #28540

Change-Id: Ifd66f04c205adc4ad6dd5ee8e79e57dce17e86bb
Reviewed-on: https://go-review.googlesource.com/c/146860Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Fixes #27914

Change-Id: Ic359a099661e959eb25d0f763ea16a6f48a3d4c7
Reviewed-on: https://go-review.googlesource.com/c/138295Reviewed-by: Andrew Bonventre <andybons@golang.org>

As discussed in golang.org/cl/28499:
Only test that all expected variables are listed in 'info locals' since
different versions of gdb print variables in different order and with
differing amount of information and formats.

Fixes #28499

Change-Id: I76627351170b5fdf2bf8cbf143e54f628b45dc4e
Reviewed-on: https://go-review.googlesource.com/c/146598Reviewed-by: Heschi Kreinick <heschi@google.com>

For many build systems, modular static analysis is most conveniently
implemented by saving analysis facts (which are analogous to export
data) in an additional section in the archive file, similar to
__PKGDEF. See golang.org/x/tools/go/analysis for an overview.

Because such sections are not object files, the linker must not
attempt to link them. This change causes the linker to skip special
sections whose name does not end with .o (and is short enough not to
be truncated).

Fixes #28429

Change-Id: I830852decf868cb017263308b114f72838032993
Reviewed-on: https://go-review.googlesource.com/c/146297
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Fixes #14210
Fixes #25941

Change-Id: Idde2d032290da3edb742b5b4f6ffeb625f05b494
Reviewed-on: https://go-review.googlesource.com/c/142884Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Change-Id: I9cb3c80ea397d964fe745b74d595df3fd8982a47
Reviewed-on: https://go-review.googlesource.com/c/146257Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Follow CL 146020 and enable RemoveAll based on Unlinkat and Openat on
freebsd.

Since the layout of syscall.Stat_t changes in FreeBSD 12, Fstatat needs
a compatibility wrapper akin to Fstatat in x/sys/unix. See CL 138595 and
CL 136816 for details.

Updates #27029

Change-Id: I8851a5b7fa658eaa6e69a1693150b16d9a68f36a
Reviewed-on: https://go-review.googlesource.com/c/146597
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>