RFR: 8328779: ZGC: `XMark::follow_object` does not need to invoke `ContinuationGCSupport::relativize_stack_chunk`

[Guoxiong Li]

Hi all, 

`XMark::follow_object` unnecessarily invokes `ContinuationGCSupport::relativize_stack_chunk`. This patch changes it to `oopDesc::is_stackChunk` instead, just like `ZMark::follow_object`.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8328779

Changes: https://git.openjdk.org/jdk/pull/18452/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18452&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328779
  Stats: 8 lines in 1 file changed: 0 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18452.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18452/head:pull/18452

PR: https://git.openjdk.org/jdk/pull/18452