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

src/platform-linux.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 716 matching lines @@
 class LinuxMutex : public Mutex {
  public:
   LinuxMutex() {
     pthread_mutexattr_t attrs;
     int result = pthread_mutexattr_init(&attrs);
     ASSERT(result == 0);
     result = pthread_mutexattr_settype(&attrs, PTHREAD_MUTEX_RECURSIVE);
     ASSERT(result == 0);
     result = pthread_mutex_init(&mutex_, &attrs);
     ASSERT(result == 0);
+    USE(result);
   }
 
   virtual ~LinuxMutex() { pthread_mutex_destroy(&mutex_); }
 
   virtual int Lock() {
     int result = pthread_mutex_lock(&mutex_);
     return result;
   }
 
   virtual int Unlock() {
@@ skipping 373 matching lines @@
 
 
 void Sampler::Stop() {
   ASSERT(IsActive());
   SignalSender::RemoveActiveSampler(this);
   SetActive(false);
 }
 
 
 } }  // namespace v8::internal