RFR: 8315948: JDK-8315818 broke Xcomp on libgraal

[Doug Simon]

This PR fixes a regression caused by [JDK-8315818](https://bugs.openjdk.org/browse/JDK-8315818). Libgraal must not unblock a thread waiting on a blocking compilation, even if libgraal itself is not yet initialized. Initialization of libgraal is not prone to the same initialization deadlocks that jargraal can cause.

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Commit messages:
 - move comment
 - do not unblock a thread waiting for a libgraal compilation even if libgraal is not yet initialized

Changes: https://git.openjdk.org/jdk/pull/15643/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15643&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315948
  Stats: 7 lines in 1 file changed: 2 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15643.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15643/head:pull/15643

PR: https://git.openjdk.org/jdk/pull/15643