/usr/mips-linux-gnu/lib/debug/libstdc++.so.6 is in libstdc++6-5-dbg-mips-cross 5.3.1-14ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/mips-linux-gnu/lib/debug/libstdc++.so.6.0.21
/usr/mips-linux-gnu/lib/debug/libstdc++.so.6.0.21