powerpc gold, work around pr17670

[Cary Coutant]

> +2014-12-04  Alan Modra  <amodra@gmail.com>
> +
> +       PR 17670
> +       * symtab.cc (Symbol::set_undefined): Remove assertion.
> +       * powerpc.cc (Target_powerpc::symval_for_branch): Don't assert
> +       on symbols defined in discarded sections, instead return false.
> +       Rearrange params, update all callers.
> +       (Target_powerpc::Branch_info::make_stub): Don't make stubs for
> +       branches to syms in discarded sections.
> +       (Global_symbol_visitor_opd::operator()): Set discarded opd syms
> +       undefined and flag as discarded.
> +       (Target_powerpc::Relocate::relocate): Localize variable.

OK, thanks!

-cary