/usr/lib/gcc/x86_64-linux-gnu/7/x32/libubsan.so is in libx32gcc-7-dev 7.3.0-16ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/7/x32/../../../../../libx32/libubsan.so.0