Allow build-time selection between ia32 and x86_64 in the GYP/Xcode Mac Chromium build

src/x64/cpu-x64.cc

 // Copyright 2009 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 15 matching lines @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // CPU specific code for x64 independent of OS goes here.
 
 #ifdef __GNUC__
 #include "third_party/valgrind/valgrind.h"
 #endif
 
 #include "v8.h"
 
+#if defined(V8_TARGET_ARCH_X64)
+
 #include "cpu.h"
 #include "macro-assembler.h"
 
 namespace v8 {
 namespace internal {
 
 void CPU::Setup() {
   CpuFeatures::Probe();
 }
 
@@ skipping 24 matching lines @@
   // To avoid Visual Studio runtime support the following code can be used
   // instead
   // __asm { int 3 }
   __debugbreak();
 #else
   asm("int $3");
 #endif
 }
 
 } }  // namespace v8::internal
+
+#endif  // V8_TARGET_ARCH_X64