CI: add Windows cross-compilation job
Now that #20657 (closed) is fixed, we can cross-compile from Linux to Windows with Hadrian:
./configure --target=x86_64-w64-mingw32 ./hadrian/build --bignum=native
We should add a CI job to check that this doesn't break.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information