Fixed `make objlink' (and `make obj' in the (non-default) OBJLINK case).

Fixed `make objlink' (and `make obj' in the (non-default) OBJLINK case).
Running them twice usually destroyed the target binary.  E.g., the
second `make objlink' in `make objlink; make; make objlink' replaced
the `cat' binary by a symlink cat@ -> /usr/obj/usr/src/bin/cat.

`ln -fs' is unusable when the target might be a symlink that resolves
to a directory.  Then -f applies to a file in the directory and not
to the symlink.  This seems to be the standard (and sometimes useful)
behaviour.