Follow golang.org/cl/50450 and add the Faccessat, Fchmodat, Fchownat,
Linkat, Mkdirat, Openat, Readlinkat, Renameat, Symlinkat and Unlinkat
functions (to conform to POSIX.1-2008) for darwin/arm and darwin/amd64.
Also add the missing AT_* constants.

Generated on Mac OS 10.11 (Darwin 15.6.0). This also regenerates the
syscall numbers for arm and arm64 akin to golang.org/cl/14151

Also fix the mksysnum call in mkall.sh to work for darwin/arm.

Fixes golang/go#14457

Change-Id: Ia3c36dbca9e6f1e4873afdfafcc4f7d8050644f9
Reviewed-on: https://go-review.googlesource.com/54271Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>