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

src/platform.h

Index: src/platform.h
===================================================================
--- src/platform.h	(revision 3297)
+++ src/platform.h	(working copy)
@@ -247,11 +247,18 @@
   // for.
   static void LogSharedLibraryAddresses();
 
+  // The return value indicates the CPU features we are sure of because of the
+  // OS.  For example MacOSX doesn't run on any x86 CPUs that don't have SSE2
+  // instructions.
+  // This is a little messy because the interpretation is subject to the cross
+  // of the CPU and the OS.  The bits in the answer correspond to the bit
+  // positions indicated by the members of the CpuFeature enum from globals.h
+  static uint64_t CpuFeaturesImpliedByPlatform();
+
   // Returns the double constant NAN
   static double nan_value();
 
   // Support runtime detection of VFP3 on ARM CPUs.
-  enum CpuFeature { VFP };
   static bool ArmCpuHasFeature(CpuFeature feature);
 
   // Returns the activation frame alignment constraint or zero if