[libc][wchar] implement wcschr

[nickdesaulniers]

In order to build cxxabi, we need wcschr. We also need wcstombs wired up to be able to test this.

See also #121183.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

In order to build cxxabi, we need wcschr. We also need `wcstombs` wired up to be able to test this.

See also #121183.

[michaelrj-google]

Described in C23 (page 458): https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=458

wcschr finds a specific character in an array of widechars.

Here's a checklist for what you'll need to do for this:

• Add the implementation of wcschr in /src/wchar
• Add tests for wcschr in /test/src/wchar
• Add wcschr to entrypoints.txt for at least x86_64 and whatever you're building on
• Add wcschr to include/wchar.yaml

[sribee8]

I would like to work on this

[michaelrj-google]

completed in #141690