tail: add WASI support by disabling notify and pid features

[sylvestre]

WASI has no file watching (inotify/kqueue) or process management APIs. Disable notify and libc on WASI, provide Observer stubs so tail compiles, and add fallback values for BACKEND and polling_help. The follow mode (-f) returns an error on WASI since it cannot work without a file watching backend.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[codspeed-hq]

Merging this PR will not alter performance

✅ 303 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing sylvestre:wasm-pr3 (71ea599) with main (1e81775)}

Footnotes

1. 46 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. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pid-pipe. tests/tail/pid-pipe is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!

[cakebaker]

There are many "never constructed" and "never used" warnings: https://github.com/uutils/coreutils/actions/runs/23864636617/job/69581278691?pr=11569#step:18:296

Update: I fixed the warnings unrelated to tail in #11599