[gold/15662] Fix a powerpc internal error in do_relax()

[Cary Coutant]

>         PR gold/15662
>         * powerpc.cc (Output_data_brlt_powerpc::reset_brlt_sizes): New
>         function.
>         (Output_data_brlt_powerpc::finalize_brlt_sizes): New function.
>         (Target_powerpc::do_relax): Call the above.

I've committed this patch for Jing.

-cary