* gc.c (mark_current_machine_context) [__mc68000__]: Update stack
  marking.
  (rb_gc_mark_machine_stack) [__mc68000__]: Also handle it here.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48065 b2dd03c8-39d4-4d8f-98ff-823fe69b080e