[jdk23u] RFR: 8334482: Shenandoah: Deadlock when safepoint is pending during nmethods iteration

[Aleksey Shipilev]

Fixes a Shenandoah deadlock.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`

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

Commit messages:
 - Backport 2aeb12ec03944c777d617d0be48982fd225b16e7

Changes: https://git.openjdk.org/jdk23u/pull/45/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk23u&pr=45&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334482
  Stats: 72 lines in 4 files changed: 25 ins; 33 del; 14 mod
  Patch: https://git.openjdk.org/jdk23u/pull/45.diff
  Fetch: git fetch https://git.openjdk.org/jdk23u.git pull/45/head:pull/45

PR: https://git.openjdk.org/jdk23u/pull/45