/usr/aarch64-linux-gnu/lib/libgnat.so is in gnat-8-aarch64-linux-gnu 8-20180414-1ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/aarch64-linux-gnu/lib/libgnat-8.so.1
/usr/aarch64-linux-gnu/lib/libgnat-8.so.1