/usr/mipsel-linux-gnu/lib64/libthread_db.so is in libc6-dev-mips64-mipsel-cross 2.23-0ubuntu3cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/mipsel-linux-gnu/lib64/libthread_db.so.1
/usr/mipsel-linux-gnu/lib64/libthread_db.so.1