Atomics overhaul.

include/ports/SkAtomics_atomic.h

+#ifndef SkAtomics_atomic_DEFINED
+#define SkAtomics_atomic_DEFINED
+
+template <typename T>
+T sk_atomic_load(const T* ptr, sk_memory_order mo) {
+    return __atomic_load_n(ptr, mo);
+}
+
+template <typename T>
+void sk_atomic_store(T* ptr, T val, sk_memory_order mo) {
+    __atomic_store_n(ptr, val, mo);
+}
+
+template <typename T>
+T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
+    return __atomic_fetch_add(ptr, val, mo);
+}
+
+template <typename T>
+bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
+                                sk_memory_order success,
+                                sk_memory_order failure) {
+    return __atomic_compare_exchange_n(ptr, expected, desired, false/*weak?*/, success, failure);
+}
+
+#endif//SkAtomics_atomic_DEFINED