RFR: 8259808: Add JFR event to detect GC locker stall

[Denghui Dong]

8259808: Add JFR event to detect GC locker stall

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Commit messages:
 - 8259808: Add JFR event to detect GC locker stall

Changes: https://git.openjdk.java.net/jdk/pull/2088/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2088&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259808
  Stats: 191 lines in 9 files changed: 191 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2088.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2088/head:pull/2088

PR: https://git.openjdk.java.net/jdk/pull/2088