Add support for building UEFI bundles on aarch64
Previously this would fail:
==> Creating UEFI executable: /boot/EFI/Linux/arch.efi
==> ERROR: UEFI stub '' not found
With this change, the bundle is built fine:
==> Image generation successful
==> Creating UEFI executable: /boot/EFI/Linux/arch.efi
-> Using UEFI stub: /usr/lib/systemd/boot/efi/linuxaa64.efi.stub
-> Using kernel image: /lib/modules/6.1.0-rc3-asahi-1-1-ARCH/vmlinuz
-> Using cmdline file: /etc/kernel/cmdline
-> Using os-release file: /etc/os-release
==> UEFI executable generation successful