RFR: 8257079: ZGC: Fold ZMark::prepare_mark() into ZMark::start()

[Per Liden]

On Wed, 25 Nov 2020 14:08:59 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Since we no longer have STW roots, ZMark::start() is now just calls perpare_mark() (and does verification), so we can just as well fold perpare_mark() into start().
>
> Marked as reviewed by ayang (Author).

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1433