RFR: Single thread-local GC state flag for all barriers

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/single-flag/webrev.01/

Please review this carefully. I did AArch64 change blindly, symmetric with x86. Taking care of SATB
check requires some specialization in g1_wb_pre, and the relevant compiler matching changes. This
would get convenient as we common the gc-state load between the safepoints, and that would not touch
G1 SATB barriers then.

Example disassembly, (0x3d8(%r15) is our flag):
  http://cr.openjdk.java.net/~shade/shenandoah/single-flag/single-flag.perfasm

Testing: hotspot_gc_shenandoah, eyeballing generated code

Thanks,
-Aleksey