RFR: 8315004: Runtime.halt() debug logging

[Masanori Yano]

I want to add a log output similar to JDK-8301627 to Runtime.halt().
To avoid double logging of Runtime.exit(), add a flag to indicate whether logging was done, and fix it so that logging is done only once.
Could someone please review this fix?

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Commit messages:
 - Merge branch 'openjdk:master' into LogRuntimeHalt
 - Merge branch 'openjdk:master' into LogRuntimeHalt
 - Add log output on Runtime.halt()
 - Add log output on Runtime.halt()

Changes: https://git.openjdk.org/jdk/pull/15426/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15426&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315004
  Stats: 21 lines in 2 files changed: 7 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/15426.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15426/head:pull/15426

PR: https://git.openjdk.org/jdk/pull/15426