/usr/share/doc/libgnatvsn6-dev-amd64-cross is in libgnatvsn6-dev-amd64-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