[GOLD] fix a current mainline g++ complaint

[Alan Modra]

On Thu, May 20, 2021 at 04:26:19PM -0700, Cary Coutant wrote:
> > I don't know enough to say gcc is correct here in the warning, or
> > whether this should be fixed in stl_pair.h.  So treat this as a heads
> > up.
> >
> >         * 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.

-- 
Alan Modra
Australia Development Lab, IBM