gitlab-ci.yml: Add build-system-debian and build-system-centos jobs

We were missing the two new targets avr-softmmu and rx-softmmu in the
gitlab-CI so far, and did not add some of the "other endianess" targets
like sh4eb-softmmu yet.
Since the current build-system-* jobs run already for a very long time,
let's do not add these missing targets there, but introduce two new
additional build jobs, one running with Debian and one running with
CentOS, and add the new targets there. Also move some targets from
the old build-system-* jobs to these new targets, to distribute the
load and reduce the runtime of the CI.

Message-Id: <20200730141326.8260-4-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>