/usr/lib/gcc-cross/i686-linux-gnu/5/libmpx.so is in libgcc-5-dev-i386-cross 5.5.0-12ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/i686-linux-gnu/5/../../../../i686-linux-gnu/lib/libmpx.so.0