/usr/x86_64-linux-gnu/lib/libquadmath.so.0 is in libquadmath0-amd64-cross 8-20180414-1ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/x86_64-linux-gnu/lib/libquadmath.so.0.0.0
/usr/x86_64-linux-gnu/lib/libquadmath.so.0.0.0