vm_core.h: preserve thread ordering

I'm hesitant to consider this as spec, but this causes a
user-visible change which could break user code.

* vm_core.h (rb_vm_living_threads_insert): preserve order
  [Bug #10660] [ruby-core:67154] [ruby-core:67159]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49036 b2dd03c8-39d4-4d8f-98ff-823fe69b080e