MIPS: Fix Add a MathFloor stub generated with TurboFan.

Port abc35080

BUG=

Review URL: https://codereview.chromium.org/1133103003

Cr-Commit-Position: refs/heads/master@{#28347}

src/mips/interface-descriptors-mips.cc

@@ -373,8 +373,8 @@ void ApiAccessorDescriptor::Initialize(CallInterfaceDescriptorData* data) {
 
 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[] = {

src/mips64/interface-descriptors-mips64.cc

@@ -373,8 +373,8 @@ void ApiAccessorDescriptor::Initialize(CallInterfaceDescriptorData* data) {
 
 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[] = {