s390 private runner CI job timing out

[Peter Maydell]

The s390 private runner CI job ubuntu-20.04-s390x-all seems to have
started timing out a lot recently. Here's an example where it passed,
but with only 53 seconds left on the clock before it would have been
killed:

https://gitlab.com/qemu-project/qemu/-/jobs/4066136770

It looks like 'make check' was about 30 minutes of the 75 minutes
total, and compilation was 45 minutes.

Any suggestions for how we can trim this down? (Presumably we
could also raise the time limit given that this is a private
runner job...)

thanks
-- PMM