/usr/share/doc/gcc-6-plugin-dev-mips64-linux-gnuabi64 is in gcc-6-plugin-dev-mips64-linux-gnuabi64 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-6-mips64-linux-gnuabi64-base