Revert "Partial revert of r21901"

src/base/atomicops_internals_x86_gcc.cc

Index: src/base/atomicops_internals_x86_gcc.cc
diff --git a/src/base/atomicops_internals_x86_gcc.cc b/src/base/atomicops_internals_x86_gcc.cc
index f2cddbe9f9e6aaa69d91ac3410e2c1e722db7e18..ffc8bce0b0ac6b22053ecb04eff6143250d70e5e 100644
--- a/src/base/atomicops_internals_x86_gcc.cc
+++ b/src/base/atomicops_internals_x86_gcc.cc
@@ -42,7 +42,6 @@ namespace base {
 // default values should hopefully be pretty safe.
 struct AtomicOps_x86CPUFeatureStruct AtomicOps_Internalx86CPUFeatures = {
   false,          // bug can't exist before process spawns multiple threads
-  false,          // no SSE2
 };
 
 } }  // namespace v8::base
@@ -88,11 +87,6 @@ void AtomicOps_Internalx86CPUFeaturesInit() {
   } 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 {