Revert "Revert "X86: Use locked add rather than mfence""

This reverts commit 0da3b9117706760e8722029f407da6d0297cc943.

Fix a compilation failure that slipped in somehow.

Change-Id: Ide8681cdc921febb296ea47aa282cc195f154049
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 92cef5f..e0ad062 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -4029,7 +4029,7 @@
    */
   switch (kind) {
     case MemBarrierKind::kAnyAny: {
-      __ mfence();
+      MemoryFence();
       break;
     }
     case MemBarrierKind::kAnyStore: