MIPS: Fix Add a MathFloor stub generated with TurboFan.

src/mips64/interface-descriptors-mips64.cc

 // Copyright 2012 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/v8.h"
 
 #if V8_TARGET_ARCH_MIPS64
 
 #include "src/interface-descriptors.h"
 
@@ skipping 355 matching lines @@
       Representation::Tagged(),    // call_data
       Representation::Tagged(),    // holder
       Representation::External(),  // api_function_address
   };
   data->Initialize(arraysize(registers), registers, representations);
 }
 
 
 void MathRoundVariantDescriptor::Initialize(CallInterfaceDescriptorData* data) {
   Register registers[] = {
-      a1,  // context
-      a2,  // math rounding function
+      cp,  // context
+      a1,  // math rounding function
       a3,  // vector slot id
   };
   Representation representations[] = {
       Representation::Tagged(),  //
       Representation::Tagged(),  //
       Representation::Tagged(),  //
   };
   data->Initialize(arraysize(registers), registers, representations);
 }
 }
 }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS64