Generate hybrid ISO images for amd64.

This keeps the existing El Torito entries for BIOS and UEFI boot code and
adds a GPT in the ISO image's System Area containing boot code for BIOS that
will load /boot/loader from the ISO filesystem and execute it. We then use
etdump to find the EFI System Partition image in the El Torito catalog and
add an entry to the GPT that allows EFI to find it.

Reviewed by: gjb, imp
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D15051