Add a mechanism to override the detected cpu features.

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 486 matching lines @@
   //   fpu, tsc, cx8, cmov, mmx, sse, sse2, fxsr, syscall
   static const uint64_t kDefaultCpuFeatures = (1 << CMOV);
 
 #ifdef DEBUG
   static bool initialized_;
 #endif
   static uint64_t supported_;
   static uint64_t found_by_runtime_probing_only_;
 
   friend class ExternalReference;
+  friend class PlatformFeatureScope;
   DISALLOW_COPY_AND_ASSIGN(CpuFeatures);
 };
 
 
 class Assembler : public AssemblerBase {
  private:
   // We check before assembling an instruction that there is sufficient
   // space to write an instruction and its relocation information.
   // The relocation writer's position must be kGap bytes above the end of
   // the generated instructions. This leaves enough space for the
@@ skipping 1142 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_