Add loongarch64 as an optional boot-test.sh arch

[imp]

Summary

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.

[emaste]

tools/boot/boot-test.sh

24–25

IMO you could just leave this text out -- no chance of forgetting about it later once loongarch lands, and -A's description is correct either way -- while out of the tree loongarch isn't supported :)

395

is it worth having a commandline option to select this too?

[imp]

tools/boot/boot-test.sh

24–25

fair.

395

Possibly. When would you need it, though? We specifically test release artifacts to make sure we don't regress from those releases.