[GOLD] fix a current mainline g++ complaint

[Cary Coutant]

> > > >         * gc.h (gc_process_relocs): Cast NULL to Relobj* in Section_id
> > > >         constructor.
> > >
> > > Please add PR gold/27815 to the ChangeLog entry. I was planning on
> > > fixing this by using nullptr instead of NULL, which would have limited
> > > our build environment to GCC 4.6 and later. I just haven't had a
> > > chance to build a GCC 12 for testing yet.
> >
> > Using nullptr succeeds with gcc-12.  I'm happy to make that change if
> > you'd prefer it that way.
>
> Sure. Thanks for checking.

After learning that it only works with the --std=g++0x option for GCC
4.6, I agree with your assessment that it's not worth it. Thanks!

-cary