[libc][wchar] implement wmemchr

[nickdesaulniers]

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

See also #121183.

[michaelrj-google]

fixed in #142640