Enable superpages on arm64 by default. These seem to be stable, having
survived multiple world and kernel builds, and of poudriere building full
package sets.

I have observed a 3% reduction in buildworld times with superpages enabled,
however further testing is needed to see if this is observed in other
workloads.

Obtained from: ABT Systems Ltd
MFC after: 1 month
Sponsored by: The FreeBSD Foundation