/usr/lib/gcc/x86_64-linux-gnu/5/libobjc_gc.so is in libobjc-5-dev 5.3.1-14ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libobjc_gc.so.4