RFR: 8309408: Thread.sleep cleanup

[Alan Bateman]

Thread.sleep has had quite a bit of churn recently to support virtual threads, add sleep(Duration), a JFR event, and the change the underlying implementation to support sub-millis precision. I think the changes have settled down now so we can do some small cleanups that came up in PR discussions. The cleanups were kicked down the road as it requires tracking down faraway tests that depend on the stack depth and the names of internal methods. The two cleanups proposed here are:

1.  Add a private sleepNanos method that creates/commits the JFR event around the sleep, this avoids duplicate code in the 3 sleep methods.
2.  Rename JVM_Sleep to JVM_SleepNanos to make it clear that it takes the sleep time in nanoseconds, esp. when Thread.sleep's parameter is milliseconds.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14303/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14303&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309408
  Stats: 75 lines in 10 files changed: 27 ins; 32 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/14303.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14303/head:pull/14303

PR: https://git.openjdk.org/jdk/pull/14303