chgrp: gate by target support

[xtqqczze]

Gate chgrp to Unix targets.

This moves the relevant target-specific handling behind the appropriate platform support checks, avoiding compilation or functionality on targets where it is not available.

Split from #12317.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.18 MB (+1004 KB, +0.65%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[codspeed-hq]

Merging this PR will not alter performance

✅ 361 untouched benchmarks
⏩ 50 skipped benchmarks¹

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_{Comparing xtqqczze:targets-chgrp (0353034) with main (e2f2570)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[xtqqczze]

@sylvestre ready