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...
Look at what those compilers are capable of compiling and to which targets, and compare it to what this compiler can do. Those are wonderful, and I have nothing but respect for them, but they aren't going to be compiling the Linux kernel.
A genuinely impressive effort, but alas, still missing some pretty critical features (const, floating point, bools, inline, anonymous structs in function args).
https://github.com/jyn514/saltwater
https://github.com/ClementTsang/rustcc
https://github.com/maekawatoshiki/rucc