* Remove the generic model; we already have generic_la32 and generic_la64 and
  pick appropriately based on bitness.
* Remove the loongarch64 model. We used this as our baseline for 64-bit, but it's
  actually pretty misleading and useless; it doesn't represent any real CPU and
  has less features than generic_la64.
* Add la64v1_0 and la64v1_1 models.
* Change our baseline CPU model for 64-bit to be la64v1_0, thus adding LSX to
  the baseline feature set.