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.
5 jobs for gitlab-ci-cross-mingw in 6 minutes and 28 seconds
Status Job ID Name Coverage
  Docker Image
passed debian-buster #9185070

00:00:11

 
  Build And Test
passed autotools #9185071

00:06:16

passed meson #9185072

00:03:20

passed meson-noglamor #9185073

00:02:41

passed mingw-cross-build #9185074

00:01:24