RFR: 8352299: GenShen: Young cycles that interrupt old cycles cannot be cancelled [v2]

[William Kemper]

> The sequence of events that creates this state:
> 1. An old collection is trying to finish marking by flushing SATB buffers with a Handshake
> 2. The regulator thread cancels old marking to start a young collection
> 3. A mutator thread shortly follows and attempts to cancel the nascent young collection
> 4. Step `3` fails (because of this bug) and cancellation reason does _not_ become `allocation failure`
> 5. The mutator thread enters a tight loop in which it retries allocations without `waiting`
> 6. The mutator thread remains in the `thread_in_vm` state and prevents the VM thread from completing step `1`.

William Kemper 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 three additional commits since the last revision:

 - Emulate single byte xchg with cmpxchg
 - Merge remote-tracking branch 'jdk/master' into fix-uncancellable-young-gc
 - Allow young cycles that interrupt old cycles to be cancelled

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24105/files
  - new: https://git.openjdk.org/jdk/pull/24105/files/9b0faf0c..adcb999b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24105&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24105&range=00-01

  Stats: 2244 lines in 56 files changed: 964 ins; 745 del; 535 mod
  Patch: https://git.openjdk.org/jdk/pull/24105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24105/head:pull/24105

PR: https://git.openjdk.org/jdk/pull/24105