[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.

-cary