RFR: 7336: Add support for new JFR event systemGC

Wed Sep 15 21:01:23 UTC 2021

`SystemGC` event brings stacktrace when a `System.gc()` is triggered
We can then correlate with GarbageCollection event with "System.gc()"
cause to this new event to be able to show stacktrace in Stacktrace
View/Flameview

<img width="893" alt="Screenshot 2021-09-15 at 22 54 03" src="https://user-images.githubusercontent.com/4610701/133508127-23bb8539-6f67-4e65-be35-6c746e08ced2.png">

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Commit messages:
 - 7336: Add support for new JFR event systemGC

Changes: https://git.openjdk.java.net/jmc/pull/307/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=307&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7336
  Stats: 158 lines in 4 files changed: 154 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jmc/pull/307.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/307/head:pull/307

PR: https://git.openjdk.java.net/jmc/pull/307