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