RFR: 8377326: [PPC64] build without C1 and C2 broken

[Martin Doerr]

Trivial fixes to get the build working when `--with-jvm-features=-compiler1,-compiler2` is configured.
`c_calling_convention` is used by the native wrapper. Some unused integer variables need to be defined.

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Commit messages:
 - 8377326: [PPC64] build without C1 and C2 broken

Changes: https://git.openjdk.org/jdk/pull/29595/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29595&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377326
  Stats: 16 lines in 1 file changed: 5 ins; 6 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/29595.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29595/head:pull/29595

PR: https://git.openjdk.org/jdk/pull/29595