py: Make int.to_bytes work on big endian machine.

Partly addresses issue #856.

py/objint.c

@@ -327,20 +327,28 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(int_from_bytes_fun_obj, 2, 3, int_fro
 STATIC MP_DEFINE_CONST_CLASSMETHOD_OBJ(int_from_bytes_obj, (const mp_obj_t)&int_from_bytes_fun_obj);
 
 STATIC mp_obj_t int_to_bytes(mp_uint_t n_args, const mp_obj_t *args) {
+    // TODO: Support long ints
+    // TODO: Support byteorder param (assumes 'little')
+    // TODO: Support signed param (assumes signed=False)
+
     mp_int_t val = mp_obj_int_get_checked(args[0]);
+    mp_int_t len = MP_OBJ_SMALL_INT_VALUE(args[1]);
 
-    uint len = MP_OBJ_SMALL_INT_VALUE(args[1]);
     byte *data;
-
-    // TODO: Support long ints
-    // TODO: Support byteorder param
-    // TODO: Support signed param
     mp_obj_t o = mp_obj_str_builder_start(&mp_type_bytes, len, &data);
     memset(data, 0, len);
+
+    if (MP_ENDIANNESS_LITTLE) {
         memcpy(data, &val, len < sizeof(mp_int_t) ? len : sizeof(mp_int_t));
+    } else {
+        while (len--) {
+            *data++ = val;
+            val >>= 8;
+        }
+    }
+
     return mp_obj_str_builder_end(o);
 }
-
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(int_to_bytes_obj, 2, 4, int_to_bytes);
 
 STATIC const mp_map_elem_t int_locals_dict_table[] = {