gas: introduce .errif and .warnif

Rather than having people resort to indirect means to issue a certain
kind of diagnostic conditionally upon an expression which can (or
should) only be evaluated when all sections were sized and all symbols
had their final values established, provide directives to directly
achieve this.

gas/write.c

@@ -2321,6 +2321,8 @@ write_object_file (void)
   resolve_local_symbol_values ();
   resolve_reloc_expr_symbols ();
 
+  evaluate_deferred_diags ();
+
 #ifdef OBJ_ELF
   if (IS_ELF)
     maybe_generate_build_notes ();