Begin migration of art::Atomic to std::atomic.

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/runtime/mirror/object.cc b/runtime/mirror/object.cc
index 04905a5..69e5a84 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -139,10 +139,10 @@
   static AtomicInteger seed(987654321 + std::time(nullptr));
   int32_t expected_value, new_value;
   do {
-    expected_value = static_cast<uint32_t>(seed.Load());
+    expected_value = static_cast<uint32_t>(seed.LoadRelaxed());
     new_value = expected_value * 1103515245 + 12345;
   } while ((expected_value & LockWord::kHashMask) == 0 ||
-      !seed.CompareAndSwap(expected_value, new_value));
+      !seed.CompareExchangeWeakRelaxed(expected_value, new_value));
   return expected_value & LockWord::kHashMask;
 }