Enable arm64 LRM builds if missing

[Impact]

 * Enable arm64 LRM builds if missing
 * nvidia-graphics-drivers-[510,515][,-server] have arm64 support in jammy+
 * If there is an LRM build for a kernel flavour which is available for arm64, it should have lrm turned on as well.

[Test Plan]

 * Check that linux-restricted-modules-* is built on arm64 and produces installable debs

[Where problems could occur]

 * build-dkms scripts in main & lrm packages should be up to date, and LRM packaging should be up to date, and nvidia drivers should be available for arm64 for this to succeed.

 * For example, at the moment we don't have nvidia drivers in focal, thus when backporting lrm to focal package.config should continue to have arm64 turned off.

[Other Info]

 * build-dkms & lrmv6 have been synced previously in https://bugs.launchpad.net/ubuntu/+source/linux-restricted-modules/+bug/1968774