syscall, cmd/go/internal/modload: add and use Access on aix

syscall, cmd/go/internal/modload: add and use Access on aix

Implement Access using Faccessat on aix following golang.org/x/sys/unix
CL 262897 and switch cmd/go/internal/modload to use it to implement
hasWritePerm.

Change-Id: I682e44737ac2bac5a203ac1c9ddd277810454426
Reviewed-on: https://go-review.googlesource.com/c/go/+/263540
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>