gitlab-ci: Split custom-runners.yml in one file per runner

gitlab-ci: Split custom-runners.yml in one file per runner

To ease maintenance, add the custom-runners/ directory and
split custom-runners.yml in 3 files, all included by the
current custom-runners.yml:
 - ubuntu-18.04-s390x.yml
 - ubuntu-20.04-aarch64.yml
 - centos-stream-8-x86_64.yml

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211115095608.2436223-1-philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20211115142915.3797652-7-alex.bennee@linaro.org>