Add code for CPU feature detection and use it to detect SSE3 and SSE 4.1.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 12828)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -272,6 +272,24 @@
 };
 
 
+class CPUFeatures : public AllStatic {
+ public:
+  static void InitOnce();
+  static bool sse3_supported();
+  static bool sse4_1_supported();
+
+ private:
+  static const uint64_t kSSE3BitMask = static_cast<uint64_t>(1) << 32;
+  static const uint64_t kSSE4_1BitMask = static_cast<uint64_t>(1) << 51;
+
+  static bool sse3_supported_;
+  static bool sse4_1_supported_;
+#ifdef DEBUG
+  static bool initialized_;
+#endif
+};
+
+
 class Assembler : public ValueObject {
  public:
   Assembler() : buffer_(), prolog_offset_(-1), comments_() { }
@@ -497,6 +515,8 @@
     cmpxchgq(address, reg);
   }
 
+  void cpuid();
+
   // Issue memory to memory move through a TMP register.
   void MoveMemoryToMemory(const Address& dst, const Address& src) {
     movq(TMP, src);