Fix compiler warnings in mmtk.c

    gc/mmtk/mmtk.c:505:1: warning: function 'rb_mmtk_gc_thread_panic_handler' could be declared with attribute 'noreturn' [-Wmissing-noreturn]
      505 | {
          | ^
    gc/mmtk/mmtk.c:987:27: warning: variable length array folded to constant array as an extension [-Wgnu-folding-constant]
      987 |         char obj_info_buf[info_size];
          |                           ^~~~~~~~~
    gc/mmtk/mmtk.c:990:34: warning: variable length array folded to constant array as an extension [-Wgnu-folding-constant]
      990 |         char parent_obj_info_buf[info_size];
          |                                  ^~~~~~~~~

Author: peterzhu2118

gc/mmtk/mmtk.c

@@ -500,6 +500,7 @@ rb_mmtk_gc_thread_bug(const char *msg, ...)
     rb_bug("rb_mmtk_gc_thread_bug");
 }
 
+RBIMPL_ATTR_NORETURN()
 static void
 rb_mmtk_gc_thread_panic_handler(void)
 {
@@ -983,7 +984,7 @@ static inline VALUE
 rb_mmtk_call_object_closure(VALUE obj, bool pin)
 {
     if (RB_UNLIKELY(RB_BUILTIN_TYPE(obj) == T_NONE)) {
-        const size_t info_size = 256;
+        enum { info_size = 256 };
         char obj_info_buf[info_size];
         rb_raw_obj_info(obj_info_buf, info_size, obj);