x/sys/unix: expose fchmodat(2) on darwin/386, darwin/arm, darwin/arm64.

[hirochachacha]

AFAIK, fchmodat(2) is introduced since OS X 10.10 and IOS 8.0.

I read the related discussion:
https://groups.google.com/forum/#!topic/golang-codereviews/Fm_icdtNKe0

I cannot understand why it is restricted to amd64.

[tklauser]

At least for darwin/386 it was added in https://golang.org/cl/50450 (golang/sys@c4489fa).

[bradfitz]

@tklauser, I take it you're going to mail the CL for arm and arm64? (fix commit message to remove double add add first)

[tklauser]

@bradfitz Yes, currently working on a CL. Thanks for the hint about commit message.

[gopherbot]

Change https://golang.org/cl/54271 mentions this issue: unix: add *at functions (POSIX.1-2008) on darwin/{arm,arm64}