/usr/lib/gcc-cross/arm-linux-gnueabihf/6/libgcc_s_sf.so is in libsfgcc-6-dev-armhf-cross 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/arm-linux-gnueabihf/6/sf/libgcc_s.so