* 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.
The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.
This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.
