Allow `Object#clone` to copy frozen status only; fix #4036

Copying all flags from the original object may overwrite the clone's
flags e.g. the embedded flag.
mrb_obj_id_m(mrb_state *mrb, mrb_value self)
{
  return mrb_fixnum_value(mrb_obj_id(self));
}

mrb_obj_id_m(mrb_state *mrb, mrb_value self)
{
  return mrb_fixnum_value(mrb_obj_id(self));
}
