RFR: 8259808: Add JFR event to detect GC locker stall [v5]

[Denghui Dong]

> GC locker will stall the operation of GC, resulting in some Java threads can not continue to run until GC locker is released, thus affecting the response time of the application. Add a JFR event to report this information is helpful to detect this issue.
> 
> For the test purpose, I add two Whitebox methods to lock/unlock critical.

Denghui Dong has updated the pull request incrementally with one additional commit since the last revision:

  Add descriptions and fix the format problem

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2088/files
  - new: https://git.openjdk.java.net/jdk/pull/2088/files/85987c58..a5d0e0a3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2088&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2088&range=03-04

  Stats: 23 lines in 2 files changed: 11 ins; 9 del; 3 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