[crac] RFR: 8358063: [CRaC] Excessive PosixAttachListener-related availability and references

[Timofei Pushkin]

Makes `PosixAttachListener` and `PosixAttachOperation` declared, defined and referenced only when they are implemented.

Sadly, I don't have access to AIX to test the buildability there.

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Commit messages:
 - Remove PosixAttachListener definition on AIX

Changes: https://git.openjdk.org/crac/pull/234/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=234&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358063
  Stats: 14 lines in 3 files changed: 6 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/crac/pull/234.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/234/head:pull/234

PR: https://git.openjdk.org/crac/pull/234