cmd/internal/obj: shuffle link.go to put declarations in original order

I think the file ended up in the order of the typedefs instead of the
order of the actual struct definitions. You can see where some of
the declarations were because some of the comments didn't move.
Put things back in the original order.

Change-Id: I0e3703008278b084b632c917cfb73bc81bdd4f23
Reviewed-on: https://go-review.googlesource.com/7743Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>