[Xtensa] add LD workaround for inconsistent linkonce sections

[Bob Wilson]

H. J. Lu wrote:
> ia64 has the same issue with unwind sections. We just fake a comdat
> group for unwind sections.

Thanks for the suggestion.  I haven't even had a chance to generate the Xtensa 
property tables into existing COMDAT groups, but once that is done, I will see 
if I can do something like what you're doing for ia64.

--Bob