runtime/cgo: add +build tags to files named for $GOOS

These are being built into the runtime/cgo for every
operating system. It doesn't seem to matter, but
restore the Go 1.3 behavior anyway.

LGTM=r
R=r, dave
CC=golang-codereviews
https://golang.org/cl/171290043

src/runtime/cgo/dragonfly.c

@@ -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 dragonfly
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't

src/runtime/cgo/freebsd.c

@@ -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 freebsd
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't

src/runtime/cgo/netbsd.c

@@ -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 netbsd
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't

src/runtime/cgo/openbsd.c

@@ -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 openbsd
+
 #include "textflag.h"
 
 // Supply environ, __progname and __guard_local, because