/usr/lib/gcc-cross/aarch64-linux-gnu/6/libatomic.so is in libgcc-6-dev-arm64-cross 6.3.0-18cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/aarch64-linux-gnu/6/../../../../aarch64-linux-gnu/lib/libatomic.so.1