unix: add AT_EACCESS on Darwin

The AT_EACCESS constant, used by faccessat syscall is present on Linux, Solaris and other BSD platforms. This commit also adds this constant on Darwin.

The constant (and the associated faccessat syscall) is part of POSIX.1-2008.

Fixes golang/go#59265.

Change-Id: Iae0ac35107cc291eba523cc88e68d3a9ddb7d01e
GitHub-Last-Rev: a6a16c4
GitHub-Pull-Request: #153
Reviewed-on: https://go-review.googlesource.com/c/sys/+/480115
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: MKuranowski

unix/types_darwin.go

@@ -304,6 +304,7 @@ const (
 	AT_REMOVEDIR        = C.AT_REMOVEDIR
 	AT_SYMLINK_FOLLOW   = C.AT_SYMLINK_FOLLOW
 	AT_SYMLINK_NOFOLLOW = C.AT_SYMLINK_NOFOLLOW
+	AT_EACCESS          = C.AT_EACCESS
 )
 
 // poll