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

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

tokio/tests/async_send_sync.rs

@@ -130,7 +130,7 @@ macro_rules! assert_value {
 macro_rules! cfg_not_wasi {
     ($($item:item)*) => {
         $(
-            #[cfg(not(target_os = "wasi"))]
+            #[cfg(not(tokio_wasi))]
             $item
         )*
     }