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

src/ia32/fast-codegen-ia32.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
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 // 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.
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 //       copyright notice, this list of conditions and the following
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 //       with the distribution.
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 //       contributors may be used to endorse or promote products derived
 //       from this software without specific prior written permission.
 //
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 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 #include "v8.h"
 
+#if defined(V8_TARGET_ARCH_IA32)
+
 #include "codegen-inl.h"
 #include "fast-codegen.h"
 #include "data-flow.h"
 #include "scopes.h"
 
 namespace v8 {
 namespace internal {
 
 #define BAILOUT(reason)                         \
   do {                                          \
@@ skipping 901 matching lines @@
 
 
 void FastCodeGenerator::VisitThisFunction(ThisFunction* expr) {
   UNREACHABLE();
 }
 
 #undef __
 
 
 } }  // namespace v8::internal
+
+#endif  // V8_TARGET_ARCH_IA32