CI: Add an x86 MinGW-w64 cross build
Cross-build for x86 MinGW-w64 only, as x64 MinGW-w64 will probably require fixing LLP64 issues.
Cross-dependencies of the X server are built from archives in the docker-image pipeline stage. This increases the container size ~335MB -> ~450MB, and the time to build it from ~5 min -> ~10 min. CI time when the container doesn't have to be rebuilt doesn't seem to increase if the additional job is run in parallel.
Doesn't build Xserver with GLX enabled, so I don't have to deal with cross-building mesa and it's dependencies.
Some X server build fixes were needed which were in a separate MR, !192 (merged)