RFR: 8323716: Only print ZGC Phase Switch events in hs_err files when running with ZGC

[Stefan Karlsson]

Don't print the ZGC Phase Switch hs_err section when the JVM is run with other GCs than ZGC.

I've tested this manually with `-XX:ErrorHandlerTest=3 -version` and verified that the section is logged when ZGC is used, and that it is not logged when G1 is used.

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Commit messages:
 - 8323716: Only print ZGC Phase Switch events in hs_err files when runing with ZGC

Changes: https://git.openjdk.org/jdk/pull/17420/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323716
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17420/head:pull/17420

PR: https://git.openjdk.org/jdk/pull/17420