Provide a `wint_t` type alias for more targets

From the `wchar.h` C standard header.

It's currently implemented only for [TEEOS](https://github.com/rust-lang/libc/blob/a5e5c0dd98ed60fca47d15fcbb33222b5c38574f/src/teeos/mod.rs#L58) & [Hurd](https://github.com/rust-lang/libc/blob/a5e5c0dd98ed60fca47d15fcbb33222b5c38574f/src/unix/hurd/mod.rs#L77). It'd be nice to also have it for at least tier 1 targets.