RFR: 8330008: [s390x] Test bit "in-memory" in case of DiagnoseSyncOnValueBasedClasses

[Amit Kumar]

It's trivial update to use `testbit` method to test the bit "in-memory"

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Commit messages:
 - use testbit

Changes: https://git.openjdk.org/jdk/pull/18709/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330008
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18709.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18709/head:pull/18709

PR: https://git.openjdk.org/jdk/pull/18709