This is a follow-up to CL 5978051.
    Use kernel cas64 helper if we can, fallback to LDREXD/STREXD if
    we are on ARMv6 or higher, and to lock-emulated cas64 if on ARMv5.
    A future CL will fix {Add,Load,Store}{Int,Uint}64 and issue 3331.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6034048

Decode AT_RANDOM, AT_HWCAP, and AT_PLATFORM.
This CL only make use of AT_RANDOM, but future CLs will make use of the others.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/5978051

Some newer Linux distributions (Ubuntu ARM at least) use a new multiarch
directory organization, where dynamic linker is no longer in the hardcoded
path in our linker.
For example, Ubuntu 12.04 ARM hardfloat places its dynamic linker at
/lib/arm-linux-gnueabihf/ld-linux.so.3

Ref: http://lackof.org/taggart/hacking/multiarch/

Also, to support Debian GNU/kFreeBSD as a FreeBSD variant, we need this capability, so it's part of issue 3533.

This CL add a new pragma (#pragma dynlinker "path") to cc.

R=iant, rsc
CC=golang-dev
https://golang.org/cl/6086043

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6185047

1. In CL 5989057, I made a mistake in the last minute change.
"MOVW.W R4, -4(SP)" should really be "MOVW.W R4, -4(R13)",
as 5l will rewrite offset for SP.
2. misc/cgo/test/issue1560.go tests for parallel sleep of 1s,
but on ARM, the deadline is frequently missed, so change sleep
time to 2s on ARM.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6202043

benchmark                     old ns/op    new ns/op    delta
BenchmarkExp3Power0x10              732          734   +0.27%
BenchmarkExp3Power0x40              834          836   +0.24%
BenchmarkExp3Power0x100            1600         1579   -1.31%
BenchmarkExp3Power0x400            3478         3417   -1.75%
BenchmarkExp3Power0x1000          19388        19229   -0.82%
BenchmarkExp3Power0x4000         160274       156881   -2.12%
BenchmarkExp3Power0x10000       1552050      1372058  -11.60%
BenchmarkExp3Power0x40000      27328710     15216920  -44.32%
BenchmarkExp3Power0x100000    612349000    131407100  -78.54%
BenchmarkExp3Power0x400000  44073524000   1122195000  -97.45%

R=golang-dev, mtj, gri, rsc
CC=golang-dev, remy
https://golang.org/cl/6176043

This adds proper note handling for Plan 9,
and fixes the issue of properly killing go procs.
Without this change, the first go proc that dies
(using runtime·exit()) would kill all the running
go procs. Proper signal handling is needed.

R=golang-dev, ality, rminnich, rsc
CC=golang-dev, john, mirtchovski
https://golang.org/cl/5617048

Instead use a new type, "Note", whose underlying
type is just a string.  This change allows us to
remove the exported os.Plan9Note type.

R=bradfitz, seed, rsc
CC=golang-dev
https://golang.org/cl/6015046

        Last Part of CL 5601044 (cgo: Linux/ARM support)
        Fixes #1880.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/5988057

        Part 4 of CL 5601044 (cgo: Linux/ARM support)

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/5989058

        Part 3 of CL 5601044 (cgo: Linux/ARM support)

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/5991066

        Part 2 of CL 5601044 (cgo: Linux/ARM support)

R=dave, rsc
CC=golang-dev
https://golang.org/cl/5989057

        Part 1 of CL 5601044 (cgo: Linux/ARM support)
        Limitation: doesn't support thumb library yet.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/5991065

All current CLs have subject lines, so we don't need to check any more.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6196044

Fixes #3450.

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/6035050

Update #3316.

R=dave, kevlar, devon.odell, rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6063053

Updates #3391.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6177045

crypto.Hash.New() changed to panicking when the hash function isn't
linked in, but crypto/x509 still expects it to return nil.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6175047

R=rsc
CC=golang-dev
https://golang.org/cl/6174049

R=golang-dev, bsiegert, rsc
CC=golang-dev
https://golang.org/cl/6158046

R=golang-dev, bradfitz, dave, rsc, lstoakes, rsc
CC=golang-dev
https://golang.org/cl/6180044

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6177043

This makes it possible to inline the prefetch of upcoming
memory addresses during garbage collection, instead of
needing to flush registers, make a function call, and
reload registers.  On garbage collection-heavy workloads,
this results in a 5% speedup.

Fixes #3493.

R=dvyukov, ken, r, dave
CC=golang-dev
https://golang.org/cl/5990066

The first bug was that tertiary ignorables had the same colElem as
implicit colElems, yielding unexpected results. The current encoding
ensures that a non-implicit colElem is never 0.  This fix uncovered
another bug of the trie that indexed incorrectly into the null block.
This was caused by an unfinished optimization that would avoid the
need to max out the most-significant bits of continuation bytes.
This bug was also present in the trie used in exp/norm and has been
fixed there as well. The appearence of the bug was rare, as the lower
blocks happened to be nearly nil.

R=r
CC=golang-dev
https://golang.org/cl/6127070

R=rsc
CC=golang-dev
https://golang.org/cl/6128051

+move zeroization out of the heap mutex

R=golang-dev, iant, rsc
CC=golang-dev
https://golang.org/cl/6094050

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6136054

R=golang-dev, r, bradfitz, r
CC=golang-dev
https://golang.org/cl/6117062

R=rsc, iant, iant, seed
CC=golang-dev
https://golang.org/cl/5608059

R=dsymonds
CC=golang-dev
https://golang.org/cl/6158043

BenchmarkEqual32                       662          159  -75.98%
BenchmarkEqual4K                     76545        13719  -82.08%
BenchmarkEqual4M                  90136700     23588870  -73.83%
BenchmarkEqual64M               2147483647   1419616000  -42.63%

BenchmarkEqual32                     48.32       201.15    4.16x
BenchmarkEqual4K                     53.51       298.56    5.58x
BenchmarkEqual4M                     46.53       177.81    3.82x
BenchmarkEqual64M                    27.12        47.27    1.74x

R=golang-dev, qyzhai, minux.ma, rsc, iant, nigeltao
CC=golang-dev
https://golang.org/cl/6118049

Fixes #3371.

R=rsc, bsiegert, r, mtj, iant
CC=golang-dev
https://golang.org/cl/6131060

R=r
CC=golang-dev
https://golang.org/cl/6136056

R=rsc, r
CC=golang-dev
https://golang.org/cl/6143043

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6127066

Fixes #3566.

R=rsc
CC=golang-dev
https://golang.org/cl/6139048

If we delay the mail sending, we can't send as the current user.
If we don't fill in the time.Time fields, datastore.Put will fail
because the zero time.Time value is out of its range.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6136053

The forwardCopy function could be re-written in asm, and the copyHuff
method could probably be rolled into huffmanBlock and copyHist, but
I'm leaving those changes for future CLs.

compress/flate benchmarks:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkDecoderBestSpeed1K                  385327       435140  +12.93%
BenchmarkDecoderBestSpeed10K                1245190      1062112  -14.70%
BenchmarkDecoderBestSpeed100K               8512365      5833680  -31.47%
BenchmarkDecoderDefaultCompression1K         382225       421301  +10.22%
BenchmarkDecoderDefaultCompression10K        867950       613890  -29.27%
BenchmarkDecoderDefaultCompression100K      5658240      2466726  -56.40%
BenchmarkDecoderBestCompression1K            383760       421634   +9.87%
BenchmarkDecoderBestCompression10K           867743       614671  -29.16%
BenchmarkDecoderBestCompression100K         5660160      2464996  -56.45%

image/png benchmarks:
benchmark                       old ns/op    new ns/op    delta
BenchmarkDecodeGray               2540834      2389624   -5.95%
BenchmarkDecodeNRGBAGradient     10052700      9534565   -5.15%
BenchmarkDecodeNRGBAOpaque        8704710      8163430   -6.22%
BenchmarkDecodePaletted           1458779      1325017   -9.17%
BenchmarkDecodeRGB                7183606      6794668   -5.41%

Wall time for Denis Cheremisov's PNG-decoding program given in
https://groups.google.com/group/golang-nuts/browse_thread/thread/22aa8a05040fdd49
Before: 3.07s
After:  2.32s
Delta:  -24%

Before profile:
Total: 304 samples
         159  52.3%  52.3%      251  82.6% compress/flate.(*decompressor).huffmanBlock
          58  19.1%  71.4%       76  25.0% compress/flate.(*decompressor).huffSym
          32  10.5%  81.9%       32  10.5% hash/adler32.update
          16   5.3%  87.2%       22   7.2% bufio.(*Reader).ReadByte
          16   5.3%  92.4%       37  12.2% compress/flate.(*decompressor).moreBits
           7   2.3%  94.7%        7   2.3% hash/crc32.update
           7   2.3%  97.0%        7   2.3% runtime.memmove
           5   1.6%  98.7%        5   1.6% scanblock
           2   0.7%  99.3%        9   3.0% runtime.copy
           1   0.3%  99.7%        1   0.3% compress/flate.(*huffmanDecoder).init

After profile:
Total: 230 samples
          59  25.7%  25.7%       70  30.4% compress/flate.(*decompressor).huffSym
          45  19.6%  45.2%       45  19.6% hash/adler32.update
          35  15.2%  60.4%       35  15.2% compress/flate.forwardCopy
          20   8.7%  69.1%      151  65.7% compress/flate.(*decompressor).huffmanBlock
          16   7.0%  76.1%       24  10.4% compress/flate.(*decompressor).moreBits
          15   6.5%  82.6%       15   6.5% runtime.memmove
          11   4.8%  87.4%       50  21.7% compress/flate.(*decompressor).copyHist
           7   3.0%  90.4%        7   3.0% hash/crc32.update
           6   2.6%  93.0%        9   3.9% bufio.(*Reader).ReadByte
           4   1.7%  94.8%        4   1.7% runtime.slicearray

R=rsc, rogpeppe, dave
CC=golang-dev, krasin
https://golang.org/cl/6127064

Fixes #3490.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6040046

       also gives clear error when bsdthread_register fails on Darwin
       Fixes #2992.

R=rsc, krautz
CC=golang-dev
https://golang.org/cl/5966067