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

src/x64/macro-assembler-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
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 //       notice, this list of conditions and the following disclaimer.
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 #include "v8.h"
 
+#if defined(V8_TARGET_ARCH_X64)
+
 #include "bootstrapper.h"
 #include "codegen-inl.h"
 #include "assembler-x64.h"
 #include "macro-assembler-x64.h"
 #include "serialize.h"
 #include "debug.h"
 
 namespace v8 {
 namespace internal {
 
@@ skipping 2719 matching lines @@
 CodePatcher::~CodePatcher() {
   // Indicate that code has changed.
   CPU::FlushICache(address_, size_);
 
   // Check that the code was patched as expected.
   ASSERT(masm_.pc_ == address_ + size_);
   ASSERT(masm_.reloc_info_writer.pos() == address_ + size_ + Assembler::kGap);
 }
 
 } }  // namespace v8::internal
+
+#endif  // V8_TARGET_ARCH_X64