Allow a platform to indicate that some CPU features are always...

src/platform-macos.cc

Index: src/platform-macos.cc
===================================================================
--- src/platform-macos.cc	(revision 3297)
+++ src/platform-macos.cc	(working copy)
@@ -244,6 +244,14 @@
 }
 
 
+uint64_t OS::CpuFeaturesImpliedByPlatform() {
+  // MacOSX requires all these to install so we can assume they are present.
+  // These constants are defined by the CPUid instructions.
+  const uint64_t one = 1;
+  return (one << SSE2) | (one << CMOV) | (one << RDTSC) | (one << CPUID);
+}
+
+
 double OS::nan_value() {
   return NAN;
 }