[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.

gold/ChangeLog

@@ -1,3 +1,8 @@
+2021-05-22  Alan Modra  <amodra@gmail.com>
+
+	PR gold/27815
+	* gc.h (gc_process_relocs): Use nullptr in Section_id constructor.
+
 2021-05-15  Alan Modra  <amodra@gmail.com>
 
 	PR 27834

gold/gc.h

@@ -247,7 +247,7 @@ gc_process_relocs(
 	      if (is_ordinary) 
 		(*secvec).push_back(Section_id(src_obj, dst_indx));
 	      else
-                (*secvec).push_back(Section_id(NULL, 0));
+		(*secvec).push_back(Section_id(nullptr, 0));
               // If the target of the relocation is an STT_SECTION symbol,
               // make a note of that by storing -1 in the symbol vector.
               if (lsym.get_st_type() == elfcpp::STT_SECTION)
@@ -329,7 +329,7 @@ gc_process_relocs(
               if (is_ordinary && dst_obj != NULL)
 		(*secvec).push_back(Section_id(dst_obj, dst_indx));
 	      else
-                (*secvec).push_back(Section_id(NULL, 0));
+		(*secvec).push_back(Section_id(nullptr, 0));
               (*symvec).push_back(gsym);
 	      (*addendvec).push_back(std::make_pair(
 					static_cast<long long>(symvalue),