[reg] Fix 13210 : libphobos2.so not being built

Accidental side effect from changes in https://github.com/D-Programming-Language/phobos/pull/2345 : target for `libphobos2.so` symlink never got referenced and thus never got executed.

This PR defines a bit more strict dependency sequence `dll -> libphobos2.so -> libbphobos2.so.0.66.0`

@MartinNowak please confirm is does not interfere with actual fix

@AndrewEdwards FYI
We do need the $(SONAME) symlink, see #2387 for a fixup.