Use futex locks on emscripten.

library/std/src/sys/unix/locks/mod.rs

@@ -2,6 +2,7 @@ cfg_if::cfg_if! {
     if #[cfg(any(
         target_os = "linux",
         target_os = "android",
+        all(target_os = "emscripten", target_feature = "atomics"),
     ))] {
         mod futex;
         mod futex_rwlock;