wasm: use build script to detect wasm (#4865)

Co-authored-by: Taiki Endo <[email protected]>

tokio/tests/task_local.rs

@@ -1,4 +1,4 @@
-#![cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
+#![cfg(all(feature = "full", not(tokio_wasi)))] // Wasi doesn't support threads
 use std::future::Future;
 use std::pin::Pin;
 use std::task::{Context, Poll};