s390x on hold

The our s390x buildd has been unstable from the beginning, making
testsuites time out for random reasons, so we had to click "retry" a
lot. Reducing the number of parallel build and test jobs from 8 to 2
made it better, but now the build queue is often 8 hours long and we
can't really do work without waiting a long time.

IBM has indicated there are currently no resources to relocate the
machine to a less busy place, so I decided to stop building s390x
packages now.

We will re-enable the architecture if the situation improves, but at
the moment I would not recommend betting on it. If that doesn't happen
before July, we'll remove the architecture from the archive.

Christoph