arm64: enable options NUMA in GENERIC

As with amd64 NUMA is required for reasonable operation on big-iron
arm64 systems and is expected to have no significant impact on small
systems. Enable it now for wider testing in advance of FreeBSD 13.0.

You can use the 'vm.ndomains' sysctl to see if multiple domains are in
use - for example (from Cavium/Marvell ThunderX2):

sysctl vm.ndomains

vm.ndomains: 2

No objection: manu
Sponsored by: The FreeBSD Foundation