Fix the MachineSignature* parameters change for turbofan unsupported platform.

   this issue is introduced by 30787d4e.
   (https://codereview.chromium.org/874693006).

   original commit message:

    Const-corrected some MachineSignature* parameters.

    Recipe: Add "const" to RawMachineAssembler constructor, fix breakage.

BUG=

Review URL: https://codereview.chromium.org/905283002

Cr-Commit-Position: refs/heads/master@{#26538}

src/compiler/linkage.cc

@@ -202,7 +202,7 @@ CallDescriptor* Linkage::GetStubCallDescriptor(
 
 
 CallDescriptor* Linkage::GetSimplifiedCDescriptor(Zone* zone,
-                                                  MachineSignature* sig) {
+                                                  const MachineSignature* sig) {
   UNIMPLEMENTED();
   return NULL;
 }