Enable building with a Cranelift-based rustc

Thanks a lot for creating and maintaining such a great set of build rules for Rust.

I would like to be able to use the cranelift-based rustc for dev builds.

From what I understand, it is possible to download the source with `http_archive` and build it using these instructions.
https://github.com/rust-lang/rustc_codegen_cranelift/tree/master?tab=readme-ov-file#building-and-testing

Sadly, I don't know if there are any differences between cmdline arguments or environment variables expected by the cranelift-based backend.

It would be awesome if cranelift were an opt-in feature behind `--compilation_mode dbg`