See the tracking issue for more details. TL;DR:
The most recent update is that aarch64-apple-darwin is now a Tier 2 platform. It is important to note that the compiler tests are not being run at this point, as CI has no way of executing them.
If you'd like to try using the new compiler, I've created some basic instructions on how to configure your project to cross-compile to aarch64-apple-darwin. If you have access to an Apple DTK, there are also instructions on how to download and install the native toolchain.