/usr/lib/gcc/x86_64-linux-gnu/5/libgcc_s_x32.so is in libx32gcc-5-dev 5.4.0-6ubuntu1~16.04.10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/5/x32/libgcc_s.so
/usr/lib/gcc/x86_64-linux-gnu/5/x32/libgcc_s.so