RFR: Upstream merge jdk/jdk -> shenandoah/jdk 2018-08-30

[Roman Kennke]

This merges up to tag jdk-12+09 (not including tagging). We shall tag it
with shenandoah-jdk-12+09 once nightlies are good.

Notable changes:

- 8209801: Rename C1_WRITE_ACCESS and C1_READ_ACCESS decorators to
ACCESS_READ and ACCESS_WRITE
- 8209667: Explicit barriers for C1/LIR
- 8206457: Code paths from oop_iterate() must use barrier-free access
- 8209841: [REDO] Refactor G1ParallelCleaningTask into shared
- 8072498: Multi-thread JNI weak reference processing
- 8210045: Allow using a subset of worker threads even when
UseDynamicNumberOfGCThreads is not set
- 8209670: CompilerThread releasing code buffer in destructor is unsafe
- 8180193: Make marking bitmap code available to other GCs


Notable problems:
- http://mail.openjdk.java.net/pipermail/build-dev/2018-August/023001.html

I included the proposed one-liner fix so that we can run tests.

- The timers in parallel-cleaning-tasks are gone. We need to rework
that. Zhengyu?

Testing: tier3_gc_shenandoah (ongoing)

http://cr.openjdk.java.net/~rkennke/upstream-jdk12-merge-2018-08-30/outgoint.txt

Ok?

Roman