Why would someone want this? (Honest question)
Another reason besides platform support, is bootstrapping and verifiable builds. rustc can only be built by a recent rustc, so it is a pain to bootstrap from source if you don't want to trust a big binary.
https://github.com/thepowersgang/mrustc is already providing that; it targets 1.54 right now, which is farther along than this project (though obviously that may not be always true in the future).