wc: count characters as bytes in single-byte locales

[darkraider01]

In single-byte locales (where MB_CUR_MAX == 1), characters should be counted as bytes for wc -m.

Previously, this was inferred by checking if the locale was named "C" or "POSIX", which does not match GNU semantics on platforms where the C locale resolves to UTF-8, nor does it handle other single-byte locales (such as ISO-8859-1).

This updates wc to detect whether the active locale encoding is genuinely single-byte via native libc locale properties (MB_CUR_MAX == 1), counting bytes directly for -m across all dispatch paths while preserving multibyte counting for UTF-8 and other multibyte locales.

Once #15139 merges, I'll rebase this follow-up ^^

Closes #14928.

[darkraider01]

Byte counting is limited to C/POSIX because other non-UTF-8 locales can still use multibyte encodings. Those locales keep their existing counting paths.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/wc/wc is no longer failing!

[codspeed]

Merging this PR will regress 8 benchmarks

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 5 improved benchmarks
❌ 8 regressed benchmarks
✅ 386 untouched benchmarks
🆕 2 new benchmarks
⏩ 54 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	three_39_bit_primes	630.5 ms	783.8 ms	-19.56%
❌	Simulation	wc_chars_large_line_count[100000]	2.3 ms	2.8 ms	-18.04%
❌	Simulation	unexpand_large_file[10]	274.8 ms	286.3 ms	-4.01%
❌	Simulation	unexpand_many_lines[100000]	131.4 ms	136.9 ms	-4%
❌	Simulation	sort_mixed_utf8_locale	80.8 ms	83.6 ms	-3.35%
❌	Simulation	sort_reverse_utf8_locale	80.6 ms	83.3 ms	-3.32%
❌	Simulation	sort_unique_utf8_locale	83.6 ms	86.3 ms	-3.19%
❌	Simulation	cksum_crc32b	33.2 ms	34.2 ms	-3.07%
⚡	Simulation	cut_fields_custom_delim	65.9 ms	51.9 ms	+27%
⚡	Simulation	cut_fields_tab	57.4 ms	45.5 ms	+26.24%
⚡	Simulation	cut_bytes	18.1 ms	15.5 ms	+16.66%
⚡	Simulation	cut_characters	25.4 ms	22.9 ms	+10.88%
⚡	Simulation	five_38_bit_primes	1.8 s	1.6 s	+8.4%
🆕	Memory	wc_chars_utf8[100000]	N/A	16.6 KB	N/A
🆕	Simulation	wc_chars_utf8[100000]	N/A	1.5 ms	N/A

Tip

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

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_{Comparing darkraider01:wc-c-locale-character-counts (0a7e171) with main (9019055)²}

Footnotes

1. 54 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 (d027897) during the generation of this report, so 9019055 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩