amd64: minor tweaks to pat decoding in sysctl vm.pmap.kernel_maps.

Decode PAT_UNCACHED.
When unknown pat mode is encountered, print the pte bits combination
instead of the index, which is always 8.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21738