runtime: export _rt0 for android

LGTM=iant, minux
R=golang-codereviews, minux, iant
CC=golang-codereviews
https://golang.org/cl/109470043

src/pkg/runtime/os_android.c

+// Copyright 2014 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 "runtime.h"
+#include "defs_GOOS_GOARCH.h"
+#include "os_GOOS.h"
+
+// Export the runtime entry point symbol.
+//
+// Used by the app package to start the Go runtime after loading
+// a shared library via JNI. See code.google.com/p/go.mobile/app.
+
+void _rt0_arm_linux1();
+#pragma cgo_export_static _rt0_arm_linux1
+#pragma cgo_export_dynamic _rt0_arm_linux1