RFR [8] [backport] Move (Java)Thread::_gc_state to lower offset to optimize barrier fast-path encoding

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/gcstate-low-8u/webrev.01/

This is the backport for the change we pushed to sh/jdk10 recently, and it seems to pass the tests
there well.

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}, eyeballing assembly

Thanks,
-Aleksey