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GH-44994: [C++][CMake] Use librt only for Linux (#44984)

### Rationale for this change

OpenBSD does not have `librt.so`.

### What changes are included in this PR?

Use `librt.so` only for Linux

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

* GitHub Issue: #44994

Lead-authored-by: Brad Smith <brad@comstyle.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>