After migrating my system to the new profile that uses SYMLINK_LIB=no, /usr/lib64/systemd directory was preserved due to sys-power/nut installing a shutdown hook in /usr/lib64/systemd/system-shutdown. Usually systemd would look for its configuration in /usr/lib/systemd, and it does not include any architecture-specific code (the shutdown script is a bash script). So I would assume it should be moved to /usr/lib/systemd.