binutils: fix -std=gnu23 compatibility wrt _Bool

GCC trunk now defaults to -std=gnu23. We return false in a few places
which can't work when true/false are a proper type (_Bool). Return NULL
where appropriate instead of false. All callers handle this appropriately.

ChangeLog:
	PR ld/32372

	* prdbg.c (visibility_name): Return NULL.

binutils/prdbg.c

@@ -2818,7 +2818,7 @@ visibility_name (enum debug_visibility visibility)
       break;
     default:
       abort ();
-      return false;
+      return NULL;
     }
   return s;
 }