RFR: 8373468: Shenandoah: Mutator may block at _gc_waiters_lock after allocation failure even block parameter is false [v3]

[Xiaolong Peng]

> There is behavior discrepancy between Shenandoah generational mode and non-generational when it handles mutator allocation failure, as stated in the description of [JDK-8373468](https://bugs.openjdk.org/browse/JDK-8373468), in non-generational, the `block` parameter may not always work.
> 
> Further looking into ShenandoahGenerationalControlThread and ShenandoahControlThread, they handle the _requested_gc_cause and ShenandoahHeap::cancelled_cause differently in other places as well, the change in this PR will minimize the gap to unify the behavior, and also fix potentially missed allocation failure notifications in some rare cases for both control threads.
> 
> ### Test
> - [x] hotspot_gc_shenandoah
> - [x] GHA (includes tier1 but no unit unit with gtest) 
> - [x] [x] gtest

Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:

  Only load _requested_gc_cause once when figuring out pending requests

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28758/files
  - new: https://git.openjdk.org/jdk/pull/28758/files/b3e18e9f..467b2746

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28758&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28758&range=01-02

  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28758.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28758/head:pull/28758

PR: https://git.openjdk.org/jdk/pull/28758