My understanding is llvm package should just provide a wrapper which take:
aarch64-w64-mingw32-{clang,clang++,...} and point to {clang,clang++,....}
Simple symbolic links or maybe a real wrapper with some options?
Then, compile-rt need to be built for windows target to get
lib/windows/libclang_rt.builtins-aarch64.a
One script doing it is available here:
https://github.com/mstorsjo/llvm-mingw/blob/master/build-compiler-rt.sh