RFR: 8232231: Use last reported wall clock time when emitting JFR per-thread CPU load events

[Robin Westberg]

Hi all,

Please review this change that ensures that the last reported wall clock time is used when emitting JFR per-thread CPU load events. Previously, it was possible that per-thread CPU usage could appear to exceed the actual usage if an intermediate event was dropped (see issue for more details).

Issue: https://bugs.openjdk.java.net/browse/JDK-8232231 
Webrev: https://cr.openjdk.java.net/~rwestberg/8232231/webrev.00/
Testing: tier1-2+5

Best regards,
Robin