gitlab-ci: Add cirrus-ci based tests for NetBSD and OpenBSD

gitlab-ci: Add cirrus-ci based tests for NetBSD and OpenBSD

Cirrus-CI provides KVM in their Linux containers, so we can also run
our VM-based NetBSD and OpenBSD build jobs there.
Since the VM installation might take a while, we only run the "help"
target on the first invocation to avoid timeouts, and then only check
the build during the next run, once the base image has been cached.
For the the build tests, we also only use very a limited set of target
CPUs since compiling in these VMs is not very fast (especially the
build on OpenBSD seems to be incredibly slow).

The jobs are marked as "manual" only, since this double-indirect setup
(with the cirrus-run script and VMs in the Cirrus-CI containers) might
fail more often than the other jobs, and since we can trigger a limited
amount of Cirrus-CI jobs at a time anyway (due to the restrictions in
the free tier of Cirrus). Thus these jobs are rather added as convenience
for contributors who would like to run the NetBSD/OpenBSD tests without
the need of downloading and installing the corresponding VM images on
their local machines.

Message-Id: <20211209103124.121942-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>