/usr/share/doc/libgfortran3-dbg-mips64-cross is in libgfortran3-dbg-mips64-cross 5.3.1-14ubuntu2cross1.
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