Did you actually look at these?

> https://github.com/jyn514/saltwater

This is just a frontend. It uses Cranelift as the backend. It's missing some fairly basic language features like bitfields and variadic functions. And if I'm reading the documentation right, it requires all the source code to be in a single file...

> https://github.com/ClementTsang/rustcc

This will compile basically no real-world code. The only supported data type is "int".

> https://github.com/maekawatoshiki/rucc

This is just a frontend. It uses LLVM as the backend.