[lworld] RFR: 8375777: [lworld] jdk/javadoc/doccheck/checks/jdkCheckLinks.java failed with missing ids

[Chen Liang]

Port the contents of https://bugs.openjdk.org/browse/JDK-8376274 over, and utilize the enhanced tag to link to the preview JLS/JVMS changeset directly from the API specs.

The `JSpec` and `ExactConversionsSupport` changes are identical to mainline. The rest are valhalla-specific fixes.

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Commit messages:
 - Fix preview spec broken links
 - 8376274: JSpec preview support and output enhancement

Changes: https://git.openjdk.org/valhalla/pull/1988/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375777
  Stats: 81 lines in 10 files changed: 49 ins; 6 del; 26 mod
  Patch: https://git.openjdk.org/valhalla/pull/1988.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1988/head:pull/1988

PR: https://git.openjdk.org/valhalla/pull/1988