main/gummiboot: enable aarch64

Merged Jake Buchholz Göktürk requested to merge (removed):gummiboot-enable-aarch64 into master

Ideally we could use gummiboot instead of the heavier-weighted grub-efi for a boot manager that hands off to the kernel's built-in EFI_STUB bootloader for Alpine cloud images. We also need to build aarch64, as major cloud providers support this arch for instances.

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