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/intrinsics_x86.cc b/compiler/optimizing/intrinsics_x86.cc
index fd454d8..74ade7c 100644
--- a/compiler/optimizing/intrinsics_x86.cc
+++ b/compiler/optimizing/intrinsics_x86.cc
@@ -2005,7 +2005,7 @@
   }
 
   if (is_volatile) {
-    __ mfence();
+    codegen->MemoryFence();
   }
 
   if (type == Primitive::kPrimNot) {