RFR: 8335289: GenShen: Whitebox breakpoint GC requests may cause assertions

[William Kemper]

Clean backport, low risk. Fixes intermittent test failure.

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Commit messages:
 - Backport 71182e240ce4f4a6e3a8773f61be6b091e2d65e9

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/63/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=63&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335289
  Stats: 26 lines in 2 files changed: 20 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/63.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/63/head:pull/63

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/63