Fix the MachineSignature* parameters change for turbofan unsupported platform.

src/compiler/linkage.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/code-stubs.h"
 #include "src/compiler.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/node.h"
 #include "src/compiler/pipeline.h"
 #include "src/scopes.h"
@@ skipping 184 matching lines @@
 CallDescriptor* Linkage::GetStubCallDescriptor(
     Isolate* isolate, Zone* zone, const CallInterfaceDescriptor& descriptor,
     int stack_parameter_count, CallDescriptor::Flags flags,
     Operator::Properties properties) {
   UNIMPLEMENTED();
   return NULL;
 }
 
 
 CallDescriptor* Linkage::GetSimplifiedCDescriptor(Zone* zone,
-                                                  MachineSignature* sig) {
+                                                  const MachineSignature* sig) {
   UNIMPLEMENTED();
   return NULL;
 }
 #endif  // !V8_TURBOFAN_BACKEND
 }
 }
 }  // namespace v8::internal::compiler