IIRC the C backend lags LLVM pretty far. Thankfully, rust tries to be compatible among a few different generations of LLVM, so maybe it's feasible.

Looks dead. No commits for almost 2 years.

I suspect you'd be much better off with a Rust MIR -> C compiler. Rust's memory and execution model is much closer to C than C is to LLVM.

The C backend was expelled from LLVM several years ago because it was too buggy and nobody wanted to fix it and since then has hardly been updated. That doesn't seem like a very pleasant route.