sort: add complete no-thread WASI fallback

[DePasqualeOrg]

I'll try to split this into more manageable pieces. Please ignore until marked as ready for review.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 361 untouched benchmarks
⏩ 50 skipped benchmarks¹

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_{Comparing DePasqualeOrg:codex/wasi-sort-no-threads (5e2a8ab) with main (dfb953d)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 (48a2013) during the generation of this report, so dfb953d was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[eduardomourar]

Just a few suggestions.

[github-actions]

Binary size comparison:

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

Total size of compared binaries: 148.94 MB (+324 KB, +0.21%)

Significant per-binary changes:
  install     1.24 MB ->    1.32 MB  (+80 KB, +6.29%)
  mkdir       1.09 MB ->    1.17 MB  (+80 KB, +7.14%)
  mkfifo      1.09 MB ->    1.16 MB  (+76 KB, +6.83%)
  mknod       1.09 MB ->    1.17 MB  (+76 KB, +6.79%)