Fix compiler warnings in mmtk.c

[peterzhu2118]

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];
      |                                  ^~~~~~~~~