Add ARM64 flatpak

[theofficialgman]

as it says in the title.

there isn't a need for multiarch for ARM64 as it isn't expected or planned for in mangohud

[gasinvein]

The problem is, we can't set per-arch sdk-extensions, and the org.freedesktop.Sdk.Compat.i386 extension isn't available for aarch64, so we can't have an aarch64 build and multilib x86_64 build at the same time. Related bug report: flatpak/flatpak-builder#381