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

src/mips/cpu-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 17 matching lines @@
 // CPU specific code for arm independent of OS goes here.
 
 #include <sys/syscall.h>
 #include <unistd.h>
 
 #ifdef __mips
 #include <asm/cachectl.h>
 #endif  // #ifdef __mips
 
 #include "v8.h"
+
+#if defined(V8_TARGET_ARCH_MIPS)
+
 #include "cpu.h"
 
 namespace v8 {
 namespace internal {
 
 void CPU::Setup() {
   // Nothing to do.
 }
 
 void CPU::FlushICache(void* start, size_t size) {
@@ skipping 12 matching lines @@
 
 
 void CPU::DebugBreak() {
 #ifdef __mips
   asm volatile("break");
 #endif  // #ifdef __mips
 }
 
 } }  // namespace v8::internal
 
+#endif  // V8_TARGET_ARCH_MIPS