RFR: Upstream merge jdk/jdk -> shenandoah/jdk 2018-08-22

[Roman Kennke]

This is stage 1 of merging jdk-12+08 into shenandoah/jdk.

Notable changes:
- 8154343: Make SATB related code available to other GCs
- 8209668: Explicit barriers for C1/assembler

It does not include any actual tags. We'll tag this (including possible
fixes and stuff that happens to go in) after nightlies look good.

http://cr.openjdk.java.net/~rkennke/jdk12-upstream-merge-2018-08-22/outgoing.txt

Testing: tier3_gc_shenandoah
Ok?

Roman