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merge revision(s) 54484: [Backport #12095]

    * 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/branches/ruby_2_2@54683 b2dd03c8-39d4-4d8f-98ff-823fe69b080e