[GOLD] Add --secure-plt option for ppc32

[Cary Coutant]

> +  DEFINE_bool(secure_plt, options::TWO_DASHES , '\0', true,
> +             N_("(PowerPC only) Use new-style PLT.  (default and forced)"),
> +             NULL);

Gold's --help will now print "(default)" automatically for this
option, so it's redundant here. What do you mean by "forced"?

-cary