pkg-bootstrap: use latest package set on arm64 stable branches

As with i386 and amd64, "latest" packages are available on stable
branches for arm64/aarch64.

Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35445

(cherry picked from commit f92e0d6acda3abd16c6d411bf90ef26c4e3c40c4)