Add `aarch64-mingw-ucrt` cross platform for Windows on ARM

The [RubyInstaller](https://rubyinstaller.org) now supports Windows on ARM.

There are build scripts to compile GCC which targets Windows on ARM: https://github.com/Windows-on-ARM-Experiments/mingw-woarm64-build
And LLVM based binaries for linux can be downloaded here: https://github.com/mstorsjo/llvm-mingw

I'll open a PR to add proper support.
