Stub out a small portion of the x64 assembler.

src/x64/assembler-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
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 // met:
 //
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+#include "v8.h"
+
+#include "macro-assembler.h"
+
+namespace v8 { namespace internal {
+
+Register no_reg = { -1 };
+
+} }  // namespace v8::internal