/usr/lib/gcc-cross/mips-linux-gnu/6/libgcc_s_64.so is in lib64gcc-6-dev-mips-cross 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/mips-linux-gnu/6/64/libgcc_s.so