RFR [10]: 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/webrev.01/

We get this automatically with sh/jdk, because there we reuse the "GC thread locals" area. We can do
the same trick in our backports, by moving the _gc_state field closer to Thread header, where GC
thread locals would be. This saves us 3 bytes per barrier fastpath check on x86.

WB check before:

     0: testb  $0x14,0x390(%r15)
    +8: jne    0x00007ff85bbb8900
   +10: ...

WB check after:

     0: testb  $0x14,0x1c(%r15)
    +5: jne    0x00007f8a57bb887a
    +7: ...

Testing: tier3_gc_shenandoah, eyeballing assembly

Thanks,
-Aleksey