RFR: 8337241: Shenandoah: Normalize include guards

[William Kemper]

Some of the include guards are not like the others.

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Commit messages:
 - Fix idiosyncratic include guards

Changes: https://git.openjdk.org/jdk/pull/20342/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20342&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337241
  Stats: 12 lines in 4 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20342.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20342/head:pull/20342

PR: https://git.openjdk.org/jdk/pull/20342