[dev.cc] runtime: migrate Android/ARM port to Go.

I tested building Go itself, but not any of go.mobile tests.

LGTM=crawshaw
R=crawshaw, rsc
CC=golang-codereviews
https://golang.org/cl/179110043

src/runtime/os_android.h

-#include "os_linux.h"

src/runtime/os_android.c → src/runtime/os_android_arm.go

@@ -2,15 +2,14 @@
 // 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"
+package runtime
+
+import _ "unsafe"
 
 // Export the runtime entry point symbol.
 //
 // Used by the app package to start the Go runtime after loading
 // a shared library via JNI. See golang.org/x/mobile/app.
 
-void _rt0_arm_linux1();
-#pragma cgo_export_static _rt0_arm_linux1
-#pragma cgo_export_dynamic _rt0_arm_linux1
+//go:cgo_export_static _rt0_arm_linux1
+//go:cgo_export_dynamic _rt0_arm_linux1

src/runtime/signal_android_386.h

-#include "signal_linux_386.h"

src/runtime/signal_android_arm.h

-#include "signal_linux_arm.h"

src/runtime/signals_android.h

-#include "signals_linux.h"