Fix bug in gold plugin interface with export-dynamic-symbol and export-dynamic-list

[Cary Coutant]

>    I am presenting this patch for review on behalf of Teresa Johnson.
>
>    Function is_visible_from_outside in plugin.cc should check for
> export-dynamic-symbol and export-dynamic-list.
>
>    Patch attached.  Please let me know if this ok.

This looks good, but it needs a ChangeLog entry. OK to apply with that.

Thanks!

-cary