esp8266/main: Mark nlr_jump_fail() as MP_FASTCODE.

It's probably not strictly needed so far, but serves as an example of
MP_FASTCODE use and may be helpful in the future.

esp8266/main.c

@@ -141,7 +141,7 @@ mp_obj_t mp_builtin_open(uint n_args, const mp_obj_t *args, mp_map_t *kwargs) {
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
 
-void nlr_jump_fail(void *val) {
+void MP_FASTCODE(nlr_jump_fail)(void *val) {
     printf("NLR jump failed\n");
     for (;;) {
     }