There are two separate ongoing projects to make a rust compiler that uses GCC as a backend (one on the gcc side adding a c++ frontend that directly reads rust, one on the rustc side to make rustc emit an intermediate format that gcc can ingest).
The long-term solution is for either of those to mature to the point where there is rust support everywhere that gcc supports.
I wonder how good a LLVM backend for these rare architectures would have to be for that to be “good enough” for the kernel team. Obviously correctness should be non-negotiable, but how important is it that the generated e.g. Alpha code is performant for somebody’s hobby?
The long-term solution is for either of those to mature to the point where there is rust support everywhere that gcc supports.