RFR: 8307106: Allow concurrent GCs to walk CLDG without ClassLoaderDataGraph_lock

[Erik Österlund]

A concurrent GC with concurrent class unloading can't currently walk the CLDG without the CLDG_lock today. We should add some synchronization code so it can do that safely. This patch adds the missing bits.

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Commit messages:
 - 8307106: Allow concurrent GCs to walk CLDG without ClassLoaderDataGraph_lock

Changes: https://git.openjdk.org/jdk/pull/13718/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307106
  Stats: 28 lines in 4 files changed: 8 ins; 12 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13718/head:pull/13718

PR: https://git.openjdk.org/jdk/pull/13718