/usr/aarch64-linux-gnu/lib/libthread_db.so is in libc6-dev-arm64-cross 2.19-0ubuntu2cross0.10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/aarch64-linux-gnu/lib/libthread_db.so.1
/usr/aarch64-linux-gnu/lib/libthread_db.so.1