RFR: 8257517: LogCompilation: Add -z to the help messages

[Eric Caspole]

I forgot to add a help message with JDK-8255965.

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Commit messages:
 - 8257517: LogCompilation: Add -z to the help messages

Changes: https://git.openjdk.java.net/jdk/pull/1640/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1640&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257517
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1640.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1640/head:pull/1640

PR: https://git.openjdk.java.net/jdk/pull/1640