CI: Add an x86 MinGW-w64 cross build
Only cross-build for x86 MinGW-w64, as x64 MinGW-w64 will probably require fixing LLP64 issues. I don't know any source for pre-built cross-dependencies of the X server, so build these from git. Don't build with GLX enabled, so I don't have to deal with cross-building mesa and it's dependencies, for the moment.
gitlab-ci-cross-mingw in 6 minutes and 28 seconds5 jobs for