[PATCH, head+2.21] Fix IRONLY visibility estimation in plugin interface, resolves GCC PR

[Cary Coutant]

>>> 2010-11-09  Dave Korn  <...
>>>
>>>      * plugin.c (is_visible_from_outside): New function.
>>>      (get_symbols): Use it.
>>>
>>>   OK for trunk and branch?
>>
>>   Ping?
>>
>
>  Just to add a sense of urgency: this patch is required for the release, as
> it resolves a PR(*) that will otherwise affect GCC 4.6.0, and is now delaying
> Tristan.  I can't self-approve it as it doesn't fall under my area
> maintainership.  The full patch is at:
>
> http://sourceware.org/ml/binutils/2010-11/msg00170.html

I can't approve, but it looks reasonable to me, and seems to match
what gold does.

-cary