arm64: generate ISO release images

Some IPMI implementations on arm64 are reportedly unable to load our
memstick installer images, but support the older ISO format. Start
generating these for arm64.

Unlike installer ISOs for other platforms, these images are UEFI-only.

Reviewed by: emaste
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26452