gitlab-ci: Build and package with MinGW cross-compilers.

This produces artifacts which can be consumed by downstream piglit
builds.

Further changes can be done to restrict this to specific branches, tags,
or even releases, as publishing artifacts on every build is likely
overkill.

Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
4 jobs for gitlab-mingw in 1 minute and 44 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Container
passed container #8007887

00:00:13

 
  Build
passed cmake #8007889

00:01:27

passed cmake-mingw #8007890

00:01:19

passed meson #8007888

00:01:18