threads: change wasm32-wasi-pthread to wasm32-wasi-threads (WebAssembly#381)

* Change `wasm32-wasi-pthread` to `wasm32-wasi-threads`

After some thought, I think that we should rename the `THREAD_MODEL=posix` build to avoid confusion. Why? Though in this project the use of this target does involve pthreads, it will not be so in other standard libraries or languages (see, e.g., rust-lang/compiler-team#574). I think it would be preferable to emphasize the "threads" Wasm-level proposal and the "wasi-threads" proposal rather than the specific details of which threading API is being exposed.

* fix: rename the `expected` output directory as well

Author: abrown