RFR: 8260267: ZGC: Reduce mark stack usage [v3]

[Per Liden]

On Thu, 29 Apr 2021 22:07:49 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review comments from StefanJ and Albert
>
> Marked as reviewed by ayang (Committer).

Thanks @albertnetymk and @kstefanj for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/3460