/usr/share/doc/libobjc4-dbg-mips64-cross is in libobjc4-dbg-mips64-cross 8-20180414-1ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-8-cross-base-ports
/usr/share/doc/gcc-8-cross-base-ports