Windows ARM is definitely on the roadmap.
WoA has some issues (namely there is a missing relocation that I need to add to lld). However, if you are going to target WoA, cross-compiling from Windows x64, using link, that I believe might be something that can be accomplished.
Windows ARM64 should be in a better state. There wasn't anything that I remember as being missing. That should be possible to build on Windows ARM64 and Windows x64.
The biggest things missing for these two platforms is mostly CI. I would love to get them added into the CI for regular builds. However, my current attention has been on Windows x86 which has had a few regressions (the last one being that FP16 support is preventing the standard library build due to a missing symbol - but if you provide that it gets through and can be used).
I imagine that since I never put WoA or WoS (Windows AArch64) through its paces as I did with Windows x86 and Windows x64, there might still be some issues to track down, but it was mostly complete.
Of course help for any of those targets is more than welcome :)