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

src/mips/disasm-mips.cc

 // Copyright 2010 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 39 matching lines @@
 #include <assert.h>
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
 #ifndef WIN32
 #include <stdint.h>
 #endif
 
 #include "v8.h"
 
+#if defined(V8_TARGET_ARCH_MIPS)
+
 #include "constants-mips.h"
 #include "disasm.h"
 #include "macro-assembler.h"
 #include "platform.h"
 
 namespace assembler {
 namespace mips {
 
 
 namespace v8i = v8::internal;
@@ skipping 705 matching lines @@
     pc += d.InstructionDecode(buffer, pc);
     fprintf(f, "%p    %08x      %s\n",
             prev_pc, *reinterpret_cast<int32_t*>(prev_pc), buffer.start());
   }
 }
 
 #undef UNSUPPORTED
 
 }  // namespace disasm
 
+#endif  // V8_TARGET_ARCH_MIPS