/usr/lib/gcc-cross/aarch64-linux-gnu/7/libobjc.so is in libobjc-7-dev-arm64-cross 7.3.0-16ubuntu3cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/aarch64-linux-gnu/7/../../../../aarch64-linux-gnu/lib/libobjc.so.4