RFR: 8315950: Shenandoah: Improvements to ShenandoahInitLogger

[William Kemper]

Changes to ShenandoahInitLogger:
* Use macro to simplify reporting of sizes
* Separate heap reporting from gc-specific reporting

Call the init logger from a virtual method to facilitate isolation of different gc modes.

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Commit messages:
 - Improve ShenandoahInitLogger

Changes: https://git.openjdk.org/jdk/pull/15646/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15646&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315950
  Stats: 29 lines in 4 files changed: 5 ins; 11 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/15646.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15646/head:pull/15646

PR: https://git.openjdk.org/jdk/pull/15646