feat(uucore): support signals feature on Haiku

[xtqqczze]

Add Haiku to the platforms supporting the signals feature and define its signal set from posix/signal.h.

RUSTC_BOOTSTRAP=1 cargo clippy -q -Zbuild-std \
  -p uucore --features signals --all-targets \
  --target x86_64-unknown-haiku

Shell script used to generate ALL_SIGNALS.

curl -fsSL
'https://raw.githubusercontent.com/haiku/haiku/refs/heads/master/headers/posix/signal.h'
|
awk '/^[[:space:]]*#define[[:space:]]+SIG[A-Z0-9_]+[[:space:]]+[0-9]+/ {
    name=$2
    sub(/^SIG/, "", name)
    sigs[$3]=name
}
END {
    print "EXIT"
    for (i=1; i<=32; i++) print sigs[i]
}'

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)

[codspeed]

Merging this PR will degrade performance by 13.18%

❌ 3 regressed benchmarks
✅ 370 untouched benchmarks
⏩ 50 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	od_default	511.8 ms	601.3 ms	-14.89%
❌	Simulation	od_chars	831.2 ms	948.2 ms	-12.34%
❌	Simulation	od_hex_bytes	870.1 ms	991.8 ms	-12.28%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing xtqqczze:uucore-feature-signals-haiku (9ceb7ef) with main (a3b3a24)²}

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. ↩

2. No successful run was found on main (17f1a5d) during the generation of this report, so a3b3a24 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[xtqqczze]

CICD / Code Coverage (ubuntu-latest, unix) (pull_request) failure tracked by #14693.