export: support overlay mount buildroot lib over system lib

This mounts build_root//lib over /usr/lib or /usr/lib32 (depending on arch), for hardware platforms which need to run X (some tests use glx..) but lack support in upstream/system mesa.

Once the mount is established, the system MUST be rebooted to umount it. For this reason, the change only targets specific platforms (mesa_ci_internal.internal_build_support.internal_vars.internal_real_hardware), instead of all CI platforms.

The overlay is mounted as read-only, so any writes to the system lib dir will fail immediately. This is done so that package updates/installs to the system libdir will fail, the alternative is they succeed but then the updated files go away on reboot (because they were written to e.g. /tmp)