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

[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.

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Commit messages:
 - Make sure nmethods are armed in pre_concurrent_startfix()

Changes: https://git.openjdk.org/jdk/pull/12194/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12194&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300915
  Stats: 13 lines in 2 files changed: 6 ins; 6 del; 1 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