Same as https://golang.org/cl/28550 but for x/sys/unix.

Updates golang/go#16937

Change-Id: Ie93df8ff1c40a7f2d98f1fb3ecf6110330bf1cbc
Reviewed-on: https://go-review.googlesource.com/28585Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>