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

src/x64/cpu-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 49 matching lines @@
 
   // If flushing of the instruction cache becomes necessary Windows has the
   // API function FlushInstructionCache.
 
   // By default, valgrind only checks the stack for writes that might need to
   // invalidate already cached translated code.  This leads to random
   // instability when code patches or moves are sometimes unnoticed.  One
   // solution is to run valgrind with --smc-check=all, but this comes at a big
   // performance cost.  We can notify valgrind to invalidate its cache.
 #ifdef VALGRIND_DISCARD_TRANSLATIONS
-  VALGRIND_DISCARD_TRANSLATIONS(start, size);
+  (void)VALGRIND_DISCARD_TRANSLATIONS(start, size);
 #endif
 }
 
 
 void CPU::DebugBreak() {
 #ifdef _MSC_VER
   // To avoid Visual Studio runtime support the following code can be used
   // instead
   // __asm { int 3 }
   __debugbreak();
 #else
   asm("int $3");
 #endif
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64