RFR: Add missing override keyword to fix MacOS (clang) builds

[William Kemper]

Adding the `override` keyword for overriding virtual methods to fix Mac builds.

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Commit messages:
 - Add more missing override keywords for MacOS build
 - Add missing override keyword to fix macos build

Changes: https://git.openjdk.java.net/shenandoah/pull/101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=101&range=00
  Stats: 13 lines in 2 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/101.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/101/head:pull/101

PR: https://git.openjdk.java.net/shenandoah/pull/101