I suspect gcc-rs will be in good working order for a few years before any kernel subsystems require a Rust compiler to build; if the legacy system can't run a recent GCC, why does it need a much-newer kernel? (e.g., how would it cope with the kernel requiring an additional GCC extension, bumping the minimum standard version of C, etc.)

I honestly suspect new architectures will be supported in LLVM before GCC nowadays; most companies are far more comfortable working with a non-GPL toolchain, and IMHO LLVM's internals are better-documented (though I've never added a new target).