esp8266: Fix garbage collector by hard-coding stack end address.

As user_init() is not a true main functions, the stack pointer captured within
is not pointing at the base of the stack. This caused gc_collect being called
with sp being higher than stack_end, causing integer overflow and crashing as
gc tried to scan almost the entire address space.

esp8266/main.c

@@ -43,7 +43,6 @@ STATIC void mp_reset(void) {
     mp_stack_set_limit(10240);
     mp_hal_init();
     gc_init(heap, heap + sizeof(heap));
-    gc_collect_init();
     mp_init();
     mp_obj_list_init(mp_sys_path, 0);
     mp_obj_list_init(mp_sys_argv, 0);