handle undefined extern vars in output_in_order

handle undefined extern vars in output_in_order

* cgraphunit.c (cgraph_order_sort_kind): New entry ORDER_VAR_UNDEF.
(output_in_order): Loop over undefined variables too.  Output them
via assemble_undefined_decl.  Skip variables that correspond to hard
registers or have value-exprs.
* varpool.c (symbol_table::output_variables): Handle undefined
variables together with defined ones.

From-SVN: r238371