/usr/lib/gcc-cross/aarch64-linux-gnu/4.8/libatomic.so is in libgcc-4.8-dev-arm64-cross 4.8.2-13ubuntu1cross0.11.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/aarch64-linux-gnu/4.8/../../../../aarch64-linux-gnu/lib/libatomic.so.1