For building Linux, you mean? I don't know whether that's the case, but I wouldn't be surprised. LLVM is missing some of the targets that Linux supports, so Linux people are probably going to want to use a Rust compiler that uses GCC as a backend (either rustc_codegen_gcc or gcc-rs) once it's sufficiently mature.
As of this moment, though, they're using the LLVM-based rustc, according to their documentation.