RFR: 8346847: [s390x] minimal build failure

[Amit Kumar]

While working on [JDK-8343884](https://bugs.openjdk.org/browse/JDK-8343884), I made sure that `OptoScheduling` remains disabled on s390x, but for that code was not guarded by C2-specific if-def (`COMPILER2`) . Which caused a build failure on minimal-vm (see JBS issue for log). This is trivial fix for that.

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Commit messages:
 - move change to c2-specific guard

Changes: https://git.openjdk.org/jdk/pull/22887/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22887&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346847
  Stats: 12 lines in 1 file changed: 6 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22887.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22887/head:pull/22887

PR: https://git.openjdk.org/jdk/pull/22887