LLVM is desirable for other reasons (LLVMpipe for example), so investing into an LLVM port is probably a better use of limited resources than improving and maintaining a retargeted rustc for GCC.

yes and no

yes, from a pure code generation aspect

no, as all conditional-compiled platform specific code is missing.

So using it with #[no_core] should work (assuming the WIP part of the backend isn't a problem). But beyond that you have to first port libcore (should be doable) and then libstd (quite a bunch of work).

Yes, that is being worked on.

The GCC backend makes Rust for Dreamcast work: https://www.dreamcast.rs/

(not relevant to Debian, but cool that it's already possible)