Rename `tests/{assembly,codegen}` into `tests/{assembly,codegen}-llvm` and ignore these testsuites if configured backend doesn't match
Follow-up of https://github.com/rust-lang/rust/pull/144125.
This PR changes `compiletest` so that `asm` tests are only run if they match the current codegen backend. To better reflect it, I renamed the `tests/ui/asm` folder into `tests/ui/asm-llvm`. Like that, we can add new asm tests for other backends if we want without needing to add extra code to `compiletest`.
Next step will be to use the new code annotations added in rust-lang/rust#144125 to ignore ui tests failing in cg_gcc until it's fixed on our side.
cc `@antoyo` `@oli-obk`
r? `@Kobzol`
a7a1618e6c