I suspect you can have this already exists if you want it bat enough, given that rust generates llvm bitcode, and there is an llvm-cbe "target" architecture that just generates C.

The results probably won't be well-optimized, though; C isn't actually very portable at this level, there's too much implementation-defined behavior that needs to be taken into account. You'll probably spend as much effort trying to robustly handle the quirks and deficiencies of some obscure platform's compiler than doing an LLVM backend for the processor...