BugLink: http://bugs.launchpad.net/bugs/1768474

The update of the SAUCE s390 Spectre patch set to the upstream version
from 4.4.130 removed the 'nogmb' kernel parameter. Add it back to retain
the original behavior.
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>