RFR: Upstream merge jdk/jdk -> shenandoah/jdk 2018-08-19

[Roman Kennke]

This is a big one. Notable changes from upstream:
- 8205921: Optimizing best-of-2 work stealing queue selection
- -Wreorder stuff
- 8205523: Explicit barriers for interpreter
- 8209118: Abstract SATBMarkQueueSet's ThreadLocalData access
- 8209396: Make PtrQueueSets not statically allocated
- 8209345: Merge SATBMarkQueueFilter into SATBMarkQueueSet
- 8209346: Refactor SATBMarkQueue filter configuration
- 8209347: SATBMarkQueue.cpp should not need jvm.h
- 8195100: Use a low latency hashtable for SymbolTable

plus Zhengyu's recent parallelcleaning stuff (hopefully to be pushed
upstream soon).

Up to tag jdk-12+7, tagged shenandoah-jdk-12+6 and shenandoah-jdk-12+7
appropriately.

http://cr.openjdk.java.net/~rkennke/upstream-merge-2018-08-19/outgoing.txt

Testing: tier3_gc_shenandoah

Ok?

Roman