Fix and improve MinGW-w64 cross-builds

This updates MinGW-w64 to the latest upstream version and also cross-builds for 32bit using MinGW-w64, because MinGW(32) doesn't seem to be maintained anymore. Also, it makes things way more consistent because we simply rely on one CRT for both 32/64 builds.

I have set up a jobset on my Hydra to check whether this breaks any non-cross-builds:

https://headcounter.org/hydra/jobset/aszlig/mingw-w64

The additional failures in 2032 are because of failed hash checks.

Any comments/suggestions before merging?