libc: locale: fix some assumptions that wchar_t cannot be signed

[kevans]

Summary

wchar_t signedness is platform-dependant, and it's actually signed on
powerpc, riscv and x86. We make some assumptions in Libc that they
aren't negative, which could lead to some invalid accesses.

Ensure that comparisons for <= UCHAR_MAX also confirm that the value is
positive, and in largesearch() let's cast it to an unsigned value in
case we actually do have a character mapped but it's just unfortunately
out of range for our signed wchar_t. We still have a chance of doing
the right thing.

Sponsored by: Klara, Inc.

[markj]

lib/libc/locale/collate.c

303

Why is this cast needed?

[kevans]

lib/libc/locale/collate.c

303

Well, 2026 Kyle doesn't remember. Reasoning through it again, I *think* my logic was that weights are explicitly unsigned in Unicode data, so naturally this should be coerced to unsigned. With fresh eyes, that doesn't make any amount of sense unless wchar_t was 16-bit signed (to avoid sign extension bugs), so I'll drop this.

[kevans]

Remove useless cast