gitlab-ci: Add cross-compiling build tests

Now that we can use all our QEMU test containers in the gitlab-CI, we can
easily add some jobs that test cross-compilation for various architectures.
There is just only small ugliness: Since the shared runners on gitlab.com
are single-threaded, we have to split each compilation job into two parts
(--disable-user and --disable-system), and exclude some additional targets,
to avoid that the jobs are running too long and hitting the timeout of 1 h.

Message-Id: <20200823111757.72002-8-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>