PATCH: Fix linker plugin support for gnu linker

[Cary Coutant]

>> I'm still not convinced that the new plugin API is necessary -- simply
>> adding the extra pass-through libraries as needed would fix the
>> problem. In either case, a gcc driver change is still necessary. That
>> by itself would fix both linkers with the 1-stage approach, with no
>> changes to either linker.
>
> To be clear, with my proposed patch, a gcc driver change is not
> required, although a similar patch could be made via a driver change.

Sorry, I was thinking that the LTO plugin needed information from the
driver about which libraries might need rescanning (for the same
reason the driver is the one to supply the pass-through options
currently). With this patch, in fact, we'd eventually be able to
remove the pass-through options from the gcc driver (once the rescan
API is known to be universally available), right?

-cary