As the stackguard was increased on AIX by CL 157117, every syscalls can
now have libcall directly on the stack. This fixes some concurrency bugs
which seems to occur when semasleep is interrupted by a SIGPROF signal.

Change-Id: I905a9618d13ef227dad6f8328b0f958f2f917a5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/172359
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>