RFR: Update remembered set during old generation mark

[Roman Kennke]

On Wed, 14 Apr 2021 21:15:10 GMT, earthling-amzn <github.com+71722661+earthling-amzn at openjdk.org> wrote:

> This change is to support the concurrent remembered set scanning work which is in progress.

Looks good to me. Thank you!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/shenandoah/pull/31