gitlab-ci.yml: Add new s390x targets to run tests with TCG and KVM accel

A new s390x z15 VM provided by IBM Community Cloud will be used to run
the s390x KVM Unit tests natively with both TCG and KVM accel options.

Signed-off-by: Marcelo Bandeira Condotta <mbandeir@redhat.com>
Message-Id: <20210208150227.178953-1-mbandeir@redhat.com>
[thuth: remove TCG job, enable all s390x tests, use "only:" section]
Signed-off-by: Thomas Huth <thuth@redhat.com>