/usr/share/doc/libgfortran3-arm64-cross is in libgfortran3-arm64-cross 4.8.2-13ubuntu1cross0.11.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-4.8-aarch64-linux-gnu-base
/usr/share/doc/gcc-4.8-aarch64-linux-gnu-base