RFR: 8262398: Shenandoah: Disable nmethod barrier and stack watermark when running with passive mode [v2]

[Zhengyu Gu]

> nmethod barrier and stack watermark allow GC not to process nmethods at GC pauses, and aim to reduce GC latency, they do not benefit STW GCs, who process nmethods at pauses anyway.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah
> - [x] tier1 with -XX:+UseShenandoahGC
> - [x] tier1 with -XX:+UseShenandoahGC -XX:ShenandoahGCMode=passive
> - [x]  tier1 with -XX:+UseShenandoahGC -XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC

Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Uses flags instead of mode
 - Merge branch 'master' into JDK-8262398-disable-barriers-stw-gc
 - JDK-8262398
 - init

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2727/files
  - new: https://git.openjdk.java.net/jdk/pull/2727/files/045c7af1..317e7e48

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2727&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2727&range=00-01

  Stats: 49768 lines in 2079 files changed: 34223 ins; 8768 del; 6777 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2727.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2727/head:pull/2727

PR: https://git.openjdk.java.net/jdk/pull/2727