[GOLD] Add debug output for ppc64 section grouping

[Cary Coutant]

> This patch adds a cleaned up version of the debug output I used to
> investigate PR20523.  I've added a new DEBUG_* flag because
> DEBUG_RELAXATION isn't useful here.  (DEBUG_RELAXATION replaces the
> normal relaxation loop with a dummy.  This code isn't even called for
> DEBUG_RELAXATION.)
>
> I also fixed a problem with the gold_debug macro.  Specifying FORMAT
> as a parameter forces you to supply something corresponding to the
> ellipsis, or to use a gcc extension that elides the comma in the
> expansion.
> See https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
>
> OK to apply?
>
>         * debug.h (DEBUG_TARGET): New.
>         (DEBUG_ALL): Add DEBUG_TARGET.
>         (gold_debug): Delete FORMAT param.
>         * powerpc.cc (Stub_control::can_add_to_stub_group): Print debug ourput.

This is OK. Thanks!

-cary