Add a MathFloor stub generated with TurboFan

src/mips64/interface-descriptors-mips64.cc

Index: src/mips64/interface-descriptors-mips64.cc
diff --git a/src/mips64/interface-descriptors-mips64.cc b/src/mips64/interface-descriptors-mips64.cc
index 50ad745eac8e568b358b24bcf8d1a56e0d36ba78..07367835d15f6201a365dbcc2b7c680f5c4ddf4a 100644
--- a/src/mips64/interface-descriptors-mips64.cc
+++ b/src/mips64/interface-descriptors-mips64.cc
@@ -363,6 +363,21 @@ void ApiAccessorDescriptor::Initialize(CallInterfaceDescriptorData* data) {
   };
   data->Initialize(arraysize(registers), registers, representations);
 }
+
+
+void MathRoundVariantDescriptor::Initialize(CallInterfaceDescriptorData* data) {
+  Register registers[] = {
+      a1,  // context
+      a2,  // math rounding function
+      a3,  // vector slot id
+  };
+  Representation representations[] = {
+      Representation::Tagged(),  //
+      Representation::Tagged(),  //
+      Representation::Tagged(),  //
+  };
+  data->Initialize(arraysize(registers), registers, representations);
+}
 }
 }  // namespace v8::internal