fix -Wunused-but-set-variable for gcc-4.6 on x64

src/x64/code-stubs-x64.cc

 // Copyright 2011 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 2511 matching lines @@
   __ bind(&setup_rest);
   // Argument 2: Previous index.
   __ movq(arg2, rbx);
 
   // Argument 1: Subject string.
 #ifdef _WIN64
   __ movq(arg1, rdi);
 #else
   // Already there in AMD64 calling convention.
   ASSERT(arg1.is(rdi));
+  USE(arg1);
 #endif
 
   // Locate the code entry and call it.
   __ addq(r11, Immediate(Code::kHeaderSize - kHeapObjectTag));
   __ call(r11);
 
   __ LeaveApiExitFrame();
 
   // Check the result.
   Label success;
@@ skipping 2739 matching lines @@
   __ Drop(1);
   __ ret(2 * kPointerSize);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64