unix: add Openat on openbsd

Change-Id: I0c9d97d7db72e74cfd5d7ba77c46726ec38b821b
Reviewed-on: https://go-review.googlesource.com/c/144172
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

@@ -266,6 +266,7 @@ func Uname(uname *Utsname) error {
 //sys	Mknod(path string, mode uint32, dev int) (err error)
 //sys	Nanosleep(time *Timespec, leftover *Timespec) (err error)
 //sys	Open(path string, mode int, perm uint32) (fd int, err error)
+//sys	Openat(dirfd int, path string, mode int, perm uint32) (fd int, err error)
 //sys	Pathconf(path string, name int) (val int, err error)
 //sys	Pread(fd int, p []byte, offset int64) (n int, err error)
 //sys	Pwrite(fd int, p []byte, offset int64) (n int, err error)
@@ -356,7 +357,6 @@ func Uname(uname *Utsname) error {
 // msgsnd
 // nfssvc
 // nnpfspioctl
-// openat
 // preadv
 // profil
 // pwritev