GitLab Gating CI: initial set of jobs, documentation and scripts

This is a mapping of Peter's "remake-merge-builds" and
"pull-buildtest" scripts, gone through some updates, adding some build
option and removing others.

The jobs currently cover the machines that the QEMU project owns, and that
are setup and ready to run jobs:

 - Ubuntu 18.04 on S390x
 - Ubuntu 20.04 on aarch64

During the development of this set of jobs, the GitLab CI was tested
with many other architectures, including ppc64, s390x and aarch64,
along with the other OSs (not included here):

 - Fedora 30
 - FreeBSD 12.1

More information can be found in the documentation itself.

Signed-off-by: Cleber Rosa <crosa@redhat.com>