os/user: fix for arm (non-cgo)

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/5504056

src/pkg/os/user/lookup_stubs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build netbsd openbsd plan9 windows
+// +build nocgo
 
 package user
 

src/pkg/os/user/lookup_unix.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build darwin freebsd linux
+// +build cgo
 
 package user