powerpc: unconditionally mark SLB zones UMA_ZONE_CONTIG

PR: 244118
Reported by: Francis Little <oggy at farscape.co.uk>
Tested by: Francis Little, Mark Millard <marklmi at yahoo.com>
Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23729