/usr/lib/gcc/x86_64-linux-gnu/6/x32/libitm.so is in libx32gcc-6-dev 6.3.0-18+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/6/x32/../../../../../libx32/libitm.so.1