PATCH: Fix linker plugin support for gnu linker

[Cary Coutant]

>> I guess this is another "unusual case":
>>
>> http://sourceware.org/bugzilla/show_bug.cgi?id=12370
>>
>> With 2 stage linking, it just works.
>
> Yes, that is another unusual case, and it's only an issue of program
> size, not correctness.  And, yes, it can be made to work with 1 stage
> linking with the plugin.

I'm not sure it's "unusual," but I agree it's a bug. If a symbol was
IRONLY, and doesn't show up in the replacement files, we should omit
it from the symbol table.

-cary