/usr/share/doc/lib64ubsan1-dbg-x32-cross is in lib64ubsan1-dbg-x32-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