/usr/lib/gcc-cross/mipsel-linux-gnu/8/libatomic.so is in libgcc-8-dev-mipsel-cross 8-20180414-1ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/mipsel-linux-gnu/8/../../../../mipsel-linux-gnu/lib/libatomic.so.1