unix: add Faccessat on OpenBSD

Change-Id: I4e3101b4689e8e94fbae6634975d10c8bb6b8a31
Reviewed-on: https://go-review.googlesource.com/118616
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_openbsd.go

@@ -201,6 +201,7 @@ func Uname(uname *Utsname) error {
 //sys	Dup(fd int) (nfd int, err error)
 //sys	Dup2(from int, to int) (err error)
 //sys	Exit(code int)
+//sys	Faccessat(dirfd int, path string, mode uint32, flags int) (err error)
 //sys	Fchdir(fd int) (err error)
 //sys	Fchflags(fd int, flags int) (err error)
 //sys	Fchmod(fd int, mode uint32) (err error)