Add compilation guard to new x64-only file, for Mac platform.

src/x64/lithium-gap-resolver-x64.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
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 //       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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+#include "v8.h"
+
+#if defined(V8_TARGET_ARCH_X64)
+
 #include "x64/lithium-gap-resolver-x64.h"
 #include "x64/lithium-codegen-x64.h"
 
 namespace v8 {
 namespace internal {
 
 LGapResolver::LGapResolver(LCodeGen* owner)
     : cgen_(owner), moves_(32) {}
 
 
@@ skipping 267 matching lines @@
       moves_[i].set_source(destination);
     } else if (other_move.Blocks(destination)) {
       moves_[i].set_source(source);
     }
   }
 }
 
 #undef __
 
 } }  // namespace v8::internal
+
+#endif  // V8_TARGET_ARCH_X64