intrinsic-test: combine rust files for faster compilation

[folkertdev]

This PR combines the rust intrinsics into number-of-cores crates, to optimize for parallelism.

It also changes the code emission strategy for tests with const-generics. Rather than duplicating test logic, we now build a big array of specialized function pointers, and iterate over this array. This logic is a bit weird to continue to match the C output order.

Locally this brings the aarch64_be docker run down from ~120s to ~85s.

[rustbot]

r? @sayantn

rustbot has assigned @sayantn.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer