PATCH: 2 stage BFD linker for LTO plugin

[Cary Coutant]

>> I'm not sure if with your patch the add_input_library or
>> add_input_file plugin hooks are completely useless (and thus
>> gold could simply ignore those at all).
>
> The plugin does need to use the add_input_file callback.  In any case
> I'm not sure it's a great idea for gold to ignore a hook, there might be
> some need for it in the future.

With this patch, the plugin won't need add_input_library, which was
added to support the pass-through option. It still needs
add_input_file, which is how the plugin inserts the newly-compiled
objects.

-cary