Revert "X86: Use locked add rather than mfence"

This reverts commit 7b3e4f99b25c31048a33a08688557b133ad345ab.

Reason: build error on sdk (linux) in git_mirror-aosp-master-with-vendor , please fix first

art/compiler/optimizing/code_generator_x86_64.cc:4032:7: error: use of
undeclared identifier 'codegen_'
      codegen_->MemoryFence();

Change-Id: I91f8542cfd944b7425d1981c35872dcdcb901e18
diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index 7a5b8db..469dd49 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -4156,7 +4156,7 @@
    */
   switch (kind) {
     case MemBarrierKind::kAnyAny: {
-      MemoryFence();
+      __ mfence();
       break;
     }
     case MemBarrierKind::kAnyStore: