arm: make libcgo build during OS X cross-compile

The gcc on OS X seems pickier about noticing
symbols that are referred to but not found during
the link of a .so file.

R=r
CC=golang-dev
https://golang.org/cl/2098047

src/libcgo/linux_arm.c

-/* unimplemented */
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "libcgo.h"
+
+void
+initcgo(void)
+{
+}
+
+void
+libcgo_sys_thread_start(ThreadStart *ts)
+{
+	// unimplemented
+	*(int*)0 = 0;
+}