deb-pkg: Add automatic support for s390x architecture

The Debian s390x architecture has 64-bit userland whereas s390 has
32-bit userland.  A 64-bit kernel can be used with either.  Now that
Debian supports multiarch and officially supports s390x, it makes more
sense to assign a 64-bit kernel package to s390x.

Reported-by: Stephen Powell <zlinuxman@wowway.com>
References: https://bugs.debian.org/750925


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>