uucore: fix C.UTF-8 locale incorrectly detected as ASCII

[mufeedali]

C.UTF-8 was treated as a byte-comparison locale because of an early return that matched on "C" before checking the ".UTF-8" encoding suffix. This caused ls to escape non-ASCII filename characters and expr to use byte-level operations on multi-byte data.

Remove the early return so C.UTF-8 and POSIX.UTF-8 are correctly detected as UTF-8 encoding. Add a locale-name guard to init_locale_collation() so sort still uses byte comparison for these locales (C/POSIX collation rules apply regardless of encoding).

[sylvestre]

could you please add tests to make sure we don't regress? thanks

[mufeedali]

Makes sense, will add

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[mufeedali]

Added regression tests.

[codspeed-hq]

Merging this PR will improve performance by 3.11%

⚡ 1 improved benchmark
✅ 318 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	sort_long_line[10000]	422.9 µs	410.2 µs	+3.11%

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_{Comparing mufeedali:locale-fix (9bd6d60) with main (b77733c)}

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. ↩