RFR: 8337241: Shenandoah: Normalize include guards

[William Kemper]

Trivial conflict.

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Commit messages:
 - Backport 4f194f10a1481cdc9df4e6338f6cabd07a34c84c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/111/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337241
  Stats: 10 lines in 4 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/111.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/111/head:pull/111

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/111