py: Make mp_sys_stdout_print object, wrapping sys.stdout for mp_print*.

So now all printing should go via either mp_plat_print or
mp_sys_stdout_print.
diff --git a/py/obj.c b/py/obj.c
index 0d318fa..eb9af9e 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -73,12 +73,7 @@
 
 void mp_obj_print(mp_obj_t o_in, mp_print_kind_t kind) {
 #if MICROPY_PY_IO
-    // defined per port; type of these is irrelevant, just need pointer
-    extern struct _mp_dummy_t mp_sys_stdout_obj;
-    mp_print_t print;
-    print.data = &mp_sys_stdout_obj;
-    print.print_strn = (mp_print_strn_t)mp_stream_write;
-    mp_obj_print_helper(&print, o_in, kind);
+    mp_obj_print_helper(&mp_sys_stdout_print, o_in, kind);
 #else
     mp_obj_print_helper(&mp_plat_print, o_in, kind);
 #endif