RFR: 8301627: System.exit and Runtime.exit debug logging

[Roger Riggs]

It can be difficult to find the cause of calls to `java.lang.System.exit(status)` and `Runtime.exit(status)` because the Java runtime exits.
The status value and stack trace are logged using the System Logger named `java.lang.Runtime` with message level `System.Logger.Level.DEBUG`.

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Commit messages:
 - Locate the System logger before taking the shutdown lock
 - Add logging of calls to Runtime.exit to the system logger "java.lang.Runtime".

Changes: https://git.openjdk.org/jdk/pull/12517/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12517&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301627
  Stats: 139 lines in 4 files changed: 139 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12517.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12517/head:pull/12517

PR: https://git.openjdk.org/jdk/pull/12517