Add SkBarriers to ports.

src/ports/SkBarriers_arm.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkBarriers_arm_DEFINED
+#define SkBarriers_arm_DEFINED
+
+static inline void sk_compiler_barrier() { asm volatile("" : : : "memory"); }
+
+template <typename T>
+T sk_acquire_load(T* ptr) {
+    T val = *ptr;
+    __sync_synchronize();  // Issue a full barrier.
+    return val;
+}
+
+template <typename T>

[bungeman-skia, 2014/05/29 18:02:52]

We should doc that T needs to be atomically read-write (that won't tear).

[mtklein, 2014/05/29 18:04:44]

Done.

+void sk_release_store(T* ptr, T val) {
+    __sync_synchronize();  // Issue a full barrier.
+    *ptr = val;
+}
+
+#endif//SkBarriers_x86_DEFINED