feat: add CLONE_NEWTIME flag for Linux

[SteveLauC]

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What does this PR do

Add CLONE_NEWTIME for Linux. Related to #2574

This PR only adds it for the GNU target, the musl support is blocked by https://github.com/rust-lang/libc/pull/4226

Checklist:

• [x] I have read CONTRIBUTING.md

• [x] I have written necessary tests and rustdoc comments

• [ ] A change log has been added if this PR modifies nix's API

  Will add the changelog entry once the musl support is added