ARM64 Dekker-pattern StoreLoad fences in java.util.concurrent + VirtualThread

[macarte]

On ARM64, volatile write (STLR/release) + volatile read (LDAR/acquire) to different addresses does NOT provide StoreLoad ordering. This breaks Dekker-like protocols where one side writes field A then reads field B, while the other writes B then reads A — both sides can miss each other's stores.

This adds U.fullFence() / VarHandle.fullFence() at all identified Dekker-pattern sites:

VirtualThread.java:

• afterYield() PARKING path: between setState(PARKED/TIMED_PARKED) and reading parkPermit (Dekker with unpark)
• afterYield() BLOCKING path: between setState(BLOCKED) and reading blockPermit (Dekker with unblock)
• afterYield() WAITING path: between setState(WAIT/TIMED_WAIT) and reading notified (Dekker with notify); fences in both untimed and timed sub-paths (adapted for tip's per-path inline checks)
• afterDone(): between setState(TERMINATED) and reading notifyAllAfterTerminate (Dekker with beforeJoin)
• unpark(): between getAndSetParkPermit(true) and reading state (Dekker with afterYield PARKING path)
• unblock(): between blockPermit=true and reading state (Dekker with afterYield BLOCKING path)

LinkedTransferQueue.java:

• xfer(): between cmpExItem CAS and reading waiter (Dekker with await() which writes waiter then reads item)

SynchronousQueue.java:

• xferLifo(): between cmpExItem CAS and reading waiter (same Dekker as LinkedTransferQueue)

AbstractQueuedSynchronizer.java:

• acquire(): between node.status=WAITING and re-reading state in tryAcquire/tryAcquireShared (Dekker with release/releaseShared)
• release(): between tryRelease state update and reading node.status in signalNext
• releaseShared(): same as release()

These fences are correctness-critical on ARM64, functionally redundant on x86 (TSO already provides StoreLoad), and appear only on non-hot paths (state transitions, not tight loops).

[macarte]

results

  Failures (all cross-platform, NOT related to ARM64 changes):

  ┌───┬──────────────────┬──────────────────────────┬──────────────────────────────────────────────┬────────────────────────┐
  │ # │ Platform         │ Test Group               │ Test                                         │ Error                  │
  ├───┼──────────────────┼──────────────────────────┼──────────────────────────────────────────────┼────────────────────────┤
  │ 1 │ linux-x64        │ jdk/tier1 part 3         │ TestSharedCloseJFR.java                      │ Exit code 1            │
  ├───┼──────────────────┼──────────────────────────┼──────────────────────────────────────────────┼────────────────────────┤
  │ 2 │ linux-x64        │ hs/tier1 compiler part 3 │ TestReturnOopSetForJFRWriteCheckpoint.java   │ IR framework exception │
  ├───┼──────────────────┼──────────────────────────┼──────────────────────────────────────────────┼────────────────────────┤
  │ 3 │ linux-x64-static │ jdk/tier1 part 3         │ (same as #1)                                 │ (same)                 │
  ├───┼──────────────────┼──────────────────────────┼──────────────────────────────────────────────┼────────────────────────┤
  │ 4 │ linux-x64-static │ hs/tier1 compiler part 3 │ (same as #2)                                 │ (same)                 │
  └───┴──────────────────┴──────────────────────────┴──────────────────────────────────────────────┴────────────────────────┘

  These are the exact same pre-existing linux-x64 flaky tests from PR #51. The PR5 Dekker fences fixed all the windows-aarch64
  timeouts — PingPong, Skynet, MultipleProducersSingleConsumerLoops, ProducerConsumerLoops, and the JVMTI GetStackTrace*StressTest
  tests are all now passing.