Add the Faccessat, Fchownat, Linkat, Mkdirat, Openat, Readlinkat,
Renameat, Symlinkat and Unlinkat functions (to conform to POSIX.1-2008)
and make Fchmodat available on all architectures, not just amd64. Also
add the missing AT_* constants.

Generated on Mac OS 10.11 (Darwin 15.6.0) for darwin/amd64 and
darwin/386 only.

Fixes golang/go#14458

Change-Id: I6d9258864383ea77f9cad058ee8ba2c728213b40
Reviewed-on: https://go-review.googlesource.com/50450Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>