runtime/cgo: add cgo build tag to C files

This makes "CGO_ENABLED=0 go list runtime/cgo" work,
which fixes the current cmd/go test failure.

Change-Id: Ia55ce3ba1dbb09f618ae5f4c8547722670360f59
Reviewed-on: https://go-review.googlesource.com/19001
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/cgo/gcc_darwin_amd64.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 cgo
+
 #include <string.h> /* for strerror */
 #include <pthread.h>
 #include <signal.h>

src/runtime/cgo/gcc_libinit.c

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build cgo
 // +build darwin dragonfly freebsd linux netbsd solaris
 // +build !ppc64,!ppc64le
 

src/runtime/cgo/gcc_setenv.c

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build cgo
 // +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 #include "libcgo.h"

src/runtime/cgo/gcc_util.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 cgo
+
 #include "libcgo.h"
 
 /* Stub for calling malloc from Go */