Retry powerpc gold stub grouping when groups prove too large

[Cary Coutant]

>> Another idea would be to automatically reduce the stub group size and
>> run another relaxation pass.
>
> Complicated somewhat by the fact that once input sections have been
> converted to relaxed sections (to hold stubs), we lack infrastructure
> to revert them back to plain input sections.

"Somewhat" -- nice understatement. Thanks for doing this!

-cary