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>
(cherry picked from commit 6c28e5f1)
4 jobs for maint-1.7 in 1 minute and 3 seconds
latest