v8 atomicops: Drop SSE2 detection if __SSE2__ is defined.

src/base/atomicops_internals_x86_gcc.cc

Index: src/base/atomicops_internals_x86_gcc.cc
===================================================================
--- src/base/atomicops_internals_x86_gcc.cc	(revision 21880)
+++ src/base/atomicops_internals_x86_gcc.cc	(working copy)
@@ -88,9 +88,10 @@
   } else {
     AtomicOps_Internalx86CPUFeatures.has_amd_lock_mb_bug = false;
   }
-
+#if !defined(__SSE2__)
   // edx bit 26 is SSE2 which we use to tell use whether we can use mfence
   AtomicOps_Internalx86CPUFeatures.has_sse2 = ((edx >> 26) & 1);
+#endif
 }
 
 class AtomicOpsx86Initializer {