[libc][wchar] implement wcsstr

[nickdesaulniers]

In order to build cxxabi, we need wcsstr. We also need wcstombs wired up to be able to test this. This should reuse our inline_memmem helper (I have a patch for this, but ran into the unit testing issue flagged above).

See also #121183.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

In order to build cxxabi, we need wcsstr. We also need wcstombs wired up to be able to test this. This should reuse our `inline_memmem` helper (I have a patch for this, but ran into the unit testing issue flagged above).

See also #121183.