[GOLD] PR27815, gold fails to build with latest GCC

...gold/gc.h:250:37: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: [-Werror]
  250 |                 (*secvec).push_back(Section_id(NULL, 0));
      |                                     ^~~~~~~~~~~~~~~~~~~

	PR gold/27815
	* gc.h (gc_process_relocs): Use nullptr in Section_id constructor.