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

tests-integration/tests/macros_select.rs

@@ -4,6 +4,7 @@ use futures::channel::oneshot;
 use futures::executor::block_on;
 use std::thread;
 
+#[cfg_attr(target_os = "wasi", ignore = "WASI: std::thread::spawn not supported")]
 #[test]
 fn join_with_select() {
     block_on(async {