Looks like it's enough to just add wrappers pointing to the
corresponding SYS_FSTATAT{,64} or SYS_NEWFSTATAT syscall. Add a test to
verify that assumption which checks that the returned stat data matches
what is returned by Stat/Lstat.

Also add the AT_NO_AUTOMOUNT constant which may be used for the flags
parmeter.

Fixes golang/go#14216

Change-Id: I73f27b483bd97d2dcf09d4b95a7957e75d194395
Reviewed-on: https://go-review.googlesource.com/79795
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>