RFR: Fix concurrent mark worker timings

[Kelvin Nilsen]

On Tue, 19 Apr 2022 23:45:45 GMT, William Kemper <wkemper at openjdk.org> wrote:

> When we added working timings for concurrent mark, they only reported the timing of the last iteration of the top level mark loop (which iteratively flushes SATB buffers on thread local handshakes). This changes adds an option to accumulate the workers' timing values.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/133