RFR: 7336: Add support for new JFR event systemGC [v2]

[Alex Macdonald]

On Thu, 16 Sep 2021 06:58:45 GMT, Jean-Philippe Bempel <jpbempel at openjdk.org> wrote:

>> `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">
>
> Jean-Philippe Bempel has updated the pull request incrementally with one additional commit since the last revision:
> 
>   formatting

Marked as reviewed by aptmac (Committer).

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