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 2478 matching lines @@
   // First four arguments are passed in registers on both Linux and Windows.
 #ifdef _WIN64
   Register arg4 = r9;
   Register arg3 = r8;
   Register arg2 = rdx;
   Register arg1 = rcx;
 #else
   Register arg4 = rcx;
   Register arg3 = rdx;
   Register arg2 = rsi;
+#ifdef DEBUG
   Register arg1 = rdi;

[William Hesse, 2011/07/19 12:16:43]

This is so asymmetric, and weird, that I would prefer a USE(arg1); line below,
instead of not declaring it.
I assume that the USE() macro also gets rid of unused variable errors?

[wingo, 2011/07/19 13:52:10]

Ah, I had not seen USE; thanks for the pointer. It's a template actually. 
Fixed.

 #endif
+#endif
 
   // Keep track on aliasing between argX defined above and the registers used.
   // rdi: subject string
   // rbx: previous index
   // rcx: encoding of subject string (1 if ascii 0 if two_byte);
   // r11: code
 
   // Argument 4: End of string data
   // Argument 3: Start of string data
   Label setup_two_byte, setup_rest;
@@ skipping 2770 matching lines @@
   __ Drop(1);
   __ ret(2 * kPointerSize);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64