I realise now that there are no official aarch64 builds. I tried to build for the Allwinner A20 device and it failed (though for a different reason). Something is off with the `gdb` configuration when building for rpi4 aarch64 which makes linking fail.
I notice that there _are_ aarch64 configs for some other platforms (e.g. Allwinner A20). I've not tried them all but do any work?
Was the build system created for LibreELEC or is it taken from somewhere else?