Upcall support for x86-64.

Sufficient to pass jni_internal_test.

Change-Id: Ia0d9b8241ab8450e04765b9c32eb6dc8fc1a8733
diff --git a/runtime/atomic.h b/runtime/atomic.h
index 2a47e46..fe9d7b8 100644
--- a/runtime/atomic.h
+++ b/runtime/atomic.h
@@ -96,7 +96,7 @@
 // quasiatomic operations that are performed on partially-overlapping
 // memory.
 class QuasiAtomic {
-#if !defined(__arm__) && !defined(__i386__)
+#if defined(__mips__) && !defined(__LP64__)
   static constexpr bool kNeedSwapMutexes = true;
 #else
   static constexpr bool kNeedSwapMutexes = false;