RFR: 8375297: ZGC: Remove obsolete O_CLOEXEC definition

[Erik Österlund]

On Wed, 14 Jan 2026 12:14:06 GMT, Jonas Norlinder <jnorlinder at openjdk.org> wrote:

> 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.

Marked as reviewed by eosterlund (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29230#pullrequestreview-3661218087