RFR: 8300915: G1: incomplete SATB because nmethod entry barriers don't get armed [v6]

[Richard Reingruber]

> The change makes sure all nmethod entry barriers are armed when a G1 concurrent marking cycle starts by doing it unconditionally in `G1ConcurrentMark::pre_concurrent_start()`. The barrier is needed to add oop constants to the SATB.
> 
> This alone would be a conservative/minimal fix but I didn't like that `CodeCache::is_gc_marking_cycle_active()` can return true after a marking cycle start is undone. I.e. `G1ConcurrentMarkThread::_state == Idle && CodeCache::is_gc_marking_cycle_active()` is possible.
> Edit: It felt like an inconsistency but maybe its actually ok not to finish the CC marking cycle when the G1 marking is undone. Please comment...
> The changes in `G1ConcurrentMark::post_concurrent_undo_start()` were made to avoid it.
> 
> 
> Testing
> 
> * Rebased this PR onto [JDK-8297487](https://bugs.openjdk.org/browse/JDK-8297487), replaced related assertions with guarantees, and then executed test/langtools:tier1 in a loop for 12h without seeing the issues reported in [JDK-8299956](https://bugs.openjdk.org/browse/JDK-8299956).
> 
> * CI testing at SAP: This includes most JCK and JTREG tiers 1-4, also in Xcomp mode, on the standard platforms and also on ppc64le.
> 
> * GHA: build failures because download of apache ant failed. `serviceability/jvmti/vthread/SuspendResumeAll/SuspendResumeAll.java` had a timeout on windows-x64. Should be unrelated. It succeeded in our CI testing.

Richard Reingruber has updated the pull request incrementally with one additional commit since the last revision:

  Rename parameter and improve comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12194/files
  - new: https://git.openjdk.org/jdk/pull/12194/files/da2162f6..55b1aa40

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12194&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12194&range=04-05

  Stats: 23 lines in 5 files changed: 3 ins; 8 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/12194.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12194/head:pull/12194

PR: https://git.openjdk.org/jdk/pull/12194