ld: 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

	* pdb.c (add_stream): Return NULL.

ld/pdb.c

@@ -178,7 +178,7 @@ add_stream (bfd *pdb, const char *name, uint16_t *stream_num)
   if (!bfd_make_writable (stream))
     {
       bfd_close (stream);
-      return false;
+      return NULL;
     }
 
   if (!pdb->archive_head)