syscall: check faccessat flags parameter on Linux

Port CL 119495 from golang.org/x/sys/unix to the syscall package.

Currently Linux faccessat(2) syscall implementation doesn't support the
flags parameter. As per the discussion in #25845, permit the same flags
as glibc [1].

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/faccessat.c;h=ea42b2303ff4b2d2d6548ea04376fb265f773436;hb=HEAD

Updates #25845

Change-Id: I132b33275a9cc72b3a97acea5482806c7f47d7f7
Reviewed-on: https://go-review.googlesource.com/120015


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>