/usr/share/doc/libgnatvsn5-dbg-alpha-cross is in libgnatvsn5-dbg-alpha-cross 5.5.0-12ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-5-cross-base-ports
/usr/share/doc/gcc-5-cross-base-ports