[libc][wchar] implement mbsinit

[nickdesaulniers]

While writing tests for c32rtomb, it looks like mbsinit could be useful. Once we add support for mbsinit, we should revisit the unit tests for c32rtomb and see if it's useful there.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

While writing tests for `c32rtomb`, it looks like `mbsinit` could be useful. Once we add support for `mbsinit`, we should revisit the unit tests for `c32rtomb` and see if it's useful there.

[michaelrj-google]

Completed in #150654