[JVMCI] Encapsulate new Thread fields for JVMCI

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>> http://cr.openjdk.java.net/~kvn/JVMCI/webrev.hotspot/src/share/vm/runtime/thread.hpp.udiff.html
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>> I thought you were only going to add 3 words to thread? (If you reordered the fields, it would be).
> I reordered the fields a bit.
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>> Having to declare this many fields inline to any class suggests it should be refactored into a different class, and a pointer to that class in the global thread class. I've made this comment before, I know. It seems all the blue in this could be a new class.
> We can make this change later if we want to.