RFR: 8342939: Building ZGC without compiler2 fails

[Thomas Schatzl]

Hi all,

  please review this change that makes ZGC compile without the C2 compiler.

The solution is to ifdef the offending C2 code, and returning `false` for the predicate that checks whether this is a C2 compilation (which can't be because there is no C2 compiled in).

Testing: manual compilation without C2, gha

Thanks,
  Thomas

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Commit messages:
 - Hi all,

Changes: https://git.openjdk.org/jdk/pull/21677/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21677&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342939
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21677.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21677/head:pull/21677

PR: https://git.openjdk.org/jdk/pull/21677