gitlab-ci: Fix ..._RUNNER_AVAILABLE variables and document them

gitlab-ci: Fix ..._RUNNER_AVAILABLE variables and document them

The patch that recently introduced the S390X_RUNNER_AVAILABLE variable
in custom-runners.yml missed that the bottom half of the file is rather
about aarch64 than s390x. Thus rename the S390X_RUNNER_AVAILABLE to
AARCH64_RUNNER_AVAILABLE in those jobs.

Finally mention both variables in our CI documentation, too.

Fixes: c5dd0f0342 ("Improve rules for the staging branch")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210730143809.717079-4-thuth@redhat.com>
[AJB: moved due to docu changes]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210806141015.2487502-5-alex.bennee@linaro.org>