contrib: rust: split rustup / rust+cargo / rust target / cargo-c targets

Each may be found on the system and ultimately we should be able to use that. Debian has packages for all of them apart from the cross compilers.

For now we don't detect the Rust cross-compiler so it's always built, via rustup in our rustup/cargo environment.

The versions of the Rust compiler or cargo-c are currently not checked. If they are present, they are used. rustup is not compiled unless it's needed.

Draft on top of !5696 (merged) so the Windows LLVM targets build correctly.