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 2d6cabeb0ba34e23056174d721591f3f58110cea..f683df9e80ad073c9748c3436b5f5053ff0f00db 100644
--- a/src/mips64/interface-descriptors-mips64.cc
+++ b/src/mips64/interface-descriptors-mips64.cc
@@ -369,6 +369,21 @@ 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