RFR 8247471: Enhance CPU load events with the actual elapsed CPU time

[Jaroslav Bachorík]

Hi all,

Here is my attempt at adding the elapsed CPU time to
ThreadCPULoadEvent and CPULoadEvent JFR events.

I have tested the implementation on Linux, MacOS and Windows and it is
working as expected.
However, I am not an expert on all of those platforms so if there is
an easier way to get the elapsed CPU time than what I am doing now I
am open to suggestions.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8247471
Webrev: http://cr.openjdk.java.net/~jbachorik/8247471

Thanks!

-JB-