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

src/builtins.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 1184 matching lines @@
 // a function (without new).
 MUST_USE_RESULT static MaybeObject* HandleApiCallAsFunctionOrConstructor(
     Isolate* isolate,
     bool is_construct_call,
     BuiltinArguments<NO_EXTRA_ARGUMENTS> args) {
   // Non-functions are never called as constructors. Even if this is an object
   // called as a constructor the delegate call is not a construct call.
   ASSERT(!CalledAsConstructor(isolate));
   Heap* heap = isolate->heap();
 
-  Handle<Object> receiver = args.at<Object>(0);
+  Handle<Object> receiver = args.receiver();
 
   // Get the object called.
-  JSObject* obj = JSObject::cast(*args.receiver());
+  JSObject* obj = JSObject::cast(*receiver);
 
   // Get the invocation callback from the function descriptor that was
   // used to create the called object.
   ASSERT(obj->map()->has_instance_call_handler());
   JSFunction* constructor = JSFunction::cast(obj->map()->constructor());
   ASSERT(constructor->shared()->IsApiFunction());
   Object* handler =
       constructor->shared()->get_api_func_data()->instance_call_handler();
   ASSERT(!handler->IsUndefined());
   CallHandlerInfo* call_data = CallHandlerInfo::cast(handler);
@@ skipping 485 matching lines @@
   return Handle<Code>(code_address);                        \
 }
 BUILTIN_LIST_C(DEFINE_BUILTIN_ACCESSOR_C)
 BUILTIN_LIST_A(DEFINE_BUILTIN_ACCESSOR_A)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 
 
 } }  // namespace v8::internal