Re-apply r4159 now with working test on x64

src/platform.h

 // Copyright 2006-2008 the V8 project authors. 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 96 matching lines @@
 #include <limits>
 #undef V8_INFINITY
 #define V8_INFINITY std::numeric_limits<double>::infinity()
 #endif
 
 #endif  // __GNUC__
 
 namespace v8 {
 namespace internal {
 
+// Use AtomicWord for a machine-sized pointer. It is assumed that
+// reads and writes of naturally aligned values of this type are atomic.
+typedef intptr_t AtomicWord;
+
 class Semaphore;
 
 double ceiling(double x);
 double modulo(double x, double y);
 
 // Forward declarations.
 class Socket;
 
 // ----------------------------------------------------------------------------
 // OS
@@ skipping 433 matching lines @@
   bool active_;
   PlatformData* data_;  // Platform specific data.
   DISALLOW_IMPLICIT_CONSTRUCTORS(Sampler);
 };
 
 #endif  // ENABLE_LOGGING_AND_PROFILING
 
 } }  // namespace v8::internal
 
 #endif  // V8_PLATFORM_H_