As a temporary measure, this limits the updater on macOS to only supporting the
`x86_64` / `x64` architecture. This ensures `aarch64` users are not
inadvertently kicked back to `x86_64`.
None of this will work as it will need tweaking (at the very least
I've not copied the origin migrator because that's had long enough)
but these are files which already existed in their current state and
so don't need re-reviewing.
