/usr/mipsisa64r6-linux-gnuabi64/lib/libthread_db.so.1 is in libc6-mips64r6-cross 2.27-3ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/mipsisa64r6-linux-gnuabi64/lib/libthread_db-1.0.so