/usr/lib/gcc/x86_64-linux-gnu/4.9/libcilkrts.so is in libgcc-4.9-dev 4.9.3-13ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/libcilkrts.so.5