The portable C back end isn’t working at the moment, and none of us has spent the time to get it working at the moment.
There is a backend for aarch64 on HEAD. It will be in the next release, which I hope to get out in the next few months. But my time is limited and there are still quite a few things to do before the release, so no promises. If you want to run off HEAD you can bootstrap using svm or cross-compiling from x86 if you have that kind of machine. Taylor posted cross-compiling instructions recently in answer to another question.