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

src/arm/codegen-arm.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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 #include "v8.h"
 
+#if defined(V8_TARGET_ARCH_ARM)
+
 #include "bootstrapper.h"
 #include "codegen-inl.h"
 #include "compiler.h"
 #include "debug.h"
 #include "ic-inl.h"
 #include "jsregexp.h"
 #include "jump-target-light-inl.h"
 #include "parser.h"
 #include "regexp-macro-assembler.h"
 #include "regexp-stack.h"
@@ skipping 9973 matching lines @@
 
   // Just jump to runtime to add the two strings.
   __ bind(&string_add_runtime);
   __ TailCallRuntime(Runtime::kStringAdd, 2, 1);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
+
+#endif  // V8_TARGET_ARCH_ARM