RFR: 8338202: Shenandoah: Improve handshake closure labels

[Aleksey Shipilev]

Currently, Shenandoah has a few handshakes that have not very clear names, "ShenandoahRendezvous". Would be good to make them more explicit.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20549/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20549&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338202
  Stats: 7 lines in 5 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20549.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20549/head:pull/20549

PR: https://git.openjdk.org/jdk/pull/20549