gitlab: disable accelerated zlib for s390x

There appears to be a bug in the s390 hardware-accelerated version of
zlib distributed with Ubuntu 20.04, which makes our test
/i386/migration/multifd/tcp/zlib hit an assertion perhaps one time in
10. Fortunately zlib provides an escape hatch where we can disable the
hardware-acceleration entirely by setting the environment variable
DFLTCC to 0. Do this on all our CI which runs on s390 hosts, both our
custom gitlab runner and also the Travis hosts.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-id: 20220321161151.3654386-1-alex.bennee@linaro.org
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>