/usr/lib/gcc-cross/mips64-linux-gnuabi64/6/libatomic.so is in libgcc-6-dev-mips64-cross 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/mips64-linux-gnuabi64/6/../../../../mips64-linux-gnuabi64/lib/libatomic.so.1