[GOLD] fix a current mainline g++ complaint

[Cary Coutant]

> 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.

-cary