RFR: 8375297: ZGC: Remove obsolete O_CLOEXEC definition

[Jonas Norlinder]

With [JDK-8375006](https://bugs.openjdk.org/browse/JDK-8375006) integrated we are no longer supporting building with on a system with kernel headers < 2.6.23 and glibc < 2.7.

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Commit messages:
 - Remove explicit define for O_CLOEXEC

Changes: https://git.openjdk.org/jdk/pull/29230/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29230&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375297
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29230.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29230/head:pull/29230

PR: https://git.openjdk.org/jdk/pull/29230