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

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

tokio/tests/rt_panic.rs

@@ -1,6 +1,6 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
-#![cfg(not(target_os = "wasi"))] // Wasi doesn't support panic recovery
+#![cfg(not(tokio_wasi))] // Wasi doesn't support panic recovery
 
 use futures::future;
 use std::error::Error;