/usr/s390x-linux-gnu/lib/librt.so is in libc6-dev-s390x-cross 2.23-0ubuntu3cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/s390x-linux-gnu/lib/librt.so.1
/usr/s390x-linux-gnu/lib/librt.so.1