Define __start/__stop symbols when there is only a dynamic def

[Alan Modra]

On Tue, Jan 30, 2018 at 07:56:59PM -0800, H.J. Lu wrote:
> I think we should check ref_dynamic, not def_dynamic, for __start/__stiop
> symbols.

Yes, I've thought about it a bit more and I think that would be
better.

-- 
Alan Modra
Australia Development Lab, IBM