net: only "build" empty.s in non-cgo mode

In cgo mode it gets passed to gcc, and on ARM
it appears that gcc does not support // comments.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/142640043

src/net/empty.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !cgo
+
 // This file is required to prevent compiler errors
 // when the package built with CGO_ENABLED=0.
 // Otherwise the compiler says: