ldapupdate: Add support for Debian multiarch
And since Fedora 28 dropped support for non-64bit, hardcode default LIBARCH as 64.
Fixes: https://pagure.io/freeipa/issue/7555
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>