[jdk16] RFR: 8258015: [JVMCI] JVMCI_lock shouldn't be held while initializing box classes

[Doug Simon]

Backport of https://bugs.openjdk.java.net/browse/JDK-8258015.

Applies cleanly.

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Commit messages:
 - 8258015: [JVMCI] JVMCI_lock shouldn't be held while initializing box classes

Changes: https://git.openjdk.java.net/jdk16/pull/1/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=1&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258015
  Stats: 26 lines in 4 files changed: 11 ins; 10 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/1.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/1/head:pull/1

PR: https://git.openjdk.java.net/jdk16/pull/1