Make page size dynamic in libkvm for arm64

To allow for a future 16k or 64k page size we need to tell libkvm which
is being used. Add a flag field in unused space in minidumphdr and use
it to signal between the different options.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34548