gitlab-ci: Build and package with MinGW cross-compilers.
This produces artifacts which can be consumed by downstream piglit.
For example, by downloading and extracting
Currently downstream piglit builds only use glut/freeglut, but there are many tests that can't run correctly without Waffle because the test window doesn't resized properly, which is something waffle doesn't suffer of.
Further changes can be done to restrict this to specific branches, tags, or even releases, as publishing artifacts on every build is likely overkill. It's relatively easy to do so, as explained in the comment. Let me know what's the general preference.
NOTE: This is part of on ongoing set of changes meant to eliminate internal VMware Jenkins instance which was used to do builds of Waffle/Piglit before FDO migrated to Gitlab.