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

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
===================================================================
--- runtime/bin/gen_snapshot.cc	(revision 12828)
+++ runtime/bin/gen_snapshot.cc	(working copy)
@@ -299,7 +299,10 @@
   // Initialize the Dart VM.
   // Note: We don't expect isolates to be created from dart code during
   // snapshot generation.
-  Dart_Initialize(NULL, NULL, NULL);
+  if (!Dart_Initialize(NULL, NULL, NULL)) {
+    fprintf(stderr, "VM initialization failed\n");
+    return 255;
+  }
 
   char* error;
   Dart_Isolate isolate = Dart_CreateIsolate(NULL, NULL, NULL, NULL, &error);