Add SkBarriers to ports.

src/ports/SkBarriers_x86.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_x86_DEFINED
+#define SkBarriers_x86_DEFINED
+
+#ifdef SK_BUILD_FOR_WIN
+#  include <intrin.h>
+static inline void sk_compiler_barrier() { _ReadWriteBarrier(); }
+#else
+static inline void sk_compiler_barrier() { asm volatile("" : : : "memory"); }
+#endif
+
+template <typename T>
+T sk_acquire_load(T* ptr) {
+    T val = *ptr;
+    // On x86, all loads are acquire loads, so we only need a compiler barrier.
+    sk_compiler_barrier();
+    return val;
+}
+
+template <typename T>
+void sk_release_store(T* ptr, T val) {
+    // On x86, all stores are release stores, so we only need a compiler barrier.
+    sk_compiler_barrier();
+    *ptr = val;
+}
+
+#endif//SkBarriers_x86_DEFINED