/usr/share/doc/libgnatprj6-dbg-s390x-cross is in libgnatprj6-dbg-s390x-cross 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-6-cross-base
/usr/share/doc/gcc-6-cross-base