build: u.h for plan9 arm

R=golang-dev, minux.ma, ality
CC=golang-dev
https://golang.org/cl/6743052

include/plan9/arm/u.h

+// Copyright 2012 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 "/arm/include/u.h"
+
+typedef char   int8;
+typedef uchar  uint8;
+typedef short  int16;
+typedef ushort uint16;
+typedef int    int32;
+typedef uint   uint32;
+typedef vlong  int64;
+typedef uvlong uint64;