RFR: 8323627: Shenandoah: Refactor init logger

[William Kemper]

Use format macros and override keyword to improvement readability.

-------------

Commit messages:
 - Simplify init logger

Changes: https://git.openjdk.org/jdk/pull/17383/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17383&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323627
  Stats: 37 lines in 2 files changed: 10 ins; 19 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/17383.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17383/head:pull/17383

PR: https://git.openjdk.org/jdk/pull/17383