* vm_core.h (rb_vm_struct): make at_exit a single linked list but
  not RArray, not to mark the registered functions by the write
  barrier.  based on the patches by Evan Phoenix.
  [ruby-core:73908] [Bug #12095]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54484 b2dd03c8-39d4-4d8f-98ff-823fe69b080e