D59889 brings up a FreeBSD/loongarch64 EFI loader, none of it in
this tree yet. Add a disabled config entry (excluded from -A, usable
via -a once the port lands) using the confirmed QEMU virt/la464
recipe; hw/loongarch/virt.c defaults to virtio-blk/net, so the
existing generic EFI+GPT builder needs no arch-specific QEMU code.
loongarch64 has never had a FreeBSD release, so
install_minimal_userland's ISO extraction can never find one. Split
it into ISO and installworld-cross-build paths, selected by a new
no_release capability, so archs without a release can still get a
minimal userland.