CI: Add a 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 22 minutes and 6 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Docker Image
passed #764659
debian-testing

00:16:58

 
  Build And Test
passed #764660
autotools-build-and-test

00:05:07

passed #764661
meson-build-and-test

00:02:43

passed #764662
mingw-cross-build

00:01:25

 
  External
passed continuous-integration/appveyor/branch