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

[Per Liden]

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().

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Commit messages:
 - 8257079: ZGC: Fold ZMark::prepare_mark() into ZMark::start()

Changes: https://git.openjdk.java.net/jdk/pull/1433/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1433&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257079
  Stats: 17 lines in 2 files changed: 5 ins; 11 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1433.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1433/head:pull/1433

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