wasm: initial support for wasm32-wasi target (#4716)

This adds initial, unstable, support for the wasm32-wasi target. Not all of Tokio's
features are supported yet as WASI's non-blocking APIs are still limited.

Refs: tokio-rs/tokio#4827

tokio/src/runtime/blocking/pool.rs

@@ -105,6 +105,7 @@ const KEEP_ALIVE: Duration = Duration::from_secs(10);
 /// Tasks will be scheduled as non-mandatory, meaning they may not get executed
 /// in case of runtime shutdown.
 #[track_caller]
+#[cfg_attr(target_os = "wasi", allow(dead_code))]
 pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
 where
     F: FnOnce() -> R + Send + 'static,