I'm not clear why you'd want to compile your rust down to C, only to then compile that C again? Surely you're better off with a single compiler invocation, and taking rust straight to object code?
I know that historically it's been easier to write a code generator than a compiler backend, but with LLVM you get the backend just as cheaply as the code gen.
Makes it easier to target embedded arches. For example, LLVM doesn't target the MSP430 from TI, but there is a gcc fork for it. Sure, you can write a new backend for LLVM, but that's a whole different ballgame.
This way it would be Rust -> LLVM IR -> C -> GCC for MSP430.
I know that historically it's been easier to write a code generator than a compiler backend, but with LLVM you get the backend just as cheaply as the code gen.