Enabling arm64 live image builds

Hi!

Live images are currently only being built for x86 machines as far as I
can see on [1] and [2].

I've tried building them on arm64 too, and everything worked perfectly
fine. I'm thus wondering whether it would be possible to build official
images for arm64 as well?

Here's the commands I've used:

$ lb config --distribution sid --updates false --bootloaders grub-efi --archive-areas 'main non-free-firmware'
$ echo task-gnome-desktop > config/package-lists/desktop.list.chroot
$ sudo lb build

The resulting image can be tested with qemu on a x86 host.

For example:

$ ISO=/var/tmp/live-image-arm64.hybrid.iso
$ cp /usr/share/AAVMF/AAVMF_VARS.fd /tmp
$ qemu-system-aarch64 -machine virt -cpu max,pauth-impdef=on -smp `nproc` -m 4G \
	-device qemu-xhci -device usb-kbd -device ramfb -device usb-tablet \
	-drive file=$ISO,format=raw,if=none,id=thumb -device usb-storage,drive=thumb,bootindex=1 \
    -drive file=/usr/share/AAVMF/AAVMF_CODE.fd,if=pflash,readonly=true \
	-drive file=/tmp/AAVMF_VARS.fd,format=raw,if=pflash \
	-netdev user,id=net0 -device virtio-net-pci,netdev=net0

Click on View -> serial0 to follow the boot process, and switch back to
ramfb after gdm has started.

Thanks!
  Emanuele

[1] https://www.debian.org/CD/live/
[2] https://cdimage.debian.org/debian-cd/current-live/