RFR: 8259808: Add JFR event to detect GC locker stall [v2]

[Denghui Dong]

On Fri, 22 Jan 2021 09:35:04 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Some comments.

Thanks for the review :)

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PR: https://git.openjdk.java.net/jdk/pull/2088