Allow compiling the wasm32-wasi std library with atomics

The issue #102157 demonstrates how currently the `-Z build-std` option
will fail when re-compiling the standard library with `RUSTFLAGS` like
`RUSTFLAGS="-C target-feature=+atomics,+bulk-memory -C
link-args=--shared-memory"`. This change attempts to resolve those build
issues by depending on the the WebAssembly `futex` module and providing
an implementation for `env_lock`. Fixes #102157.

library/std/src/sys/wasi/mod.rs

@@ -25,6 +25,9 @@
 pub mod env;
 pub mod fd;
 pub mod fs;
+#[allow(unused)]
+#[path = "../wasm/atomics/futex.rs"]
+pub mod futex;
 pub mod io;
 #[path = "../unsupported/locks/mod.rs"]
 pub mod locks;

library/std/src/sys/wasi/os.rs

@@ -24,10 +24,15 @@ mod libc {
     }
 }
 
-#[cfg(not(target_feature = "atomics"))]
 pub unsafe fn env_lock() -> impl Any {
-    // No need for a lock if we're single-threaded, but this function will need
-    // to get implemented for multi-threaded scenarios
+    cfg_if::cfg_if! {
+        if #[cfg(target_feature = "atomics")] {
+            todo!()
+        } else {
+            // No need for a lock if we're single-threaded, but this function will need
+            // to get implemented for multi-threaded scenarios
+        }
+    }
 }
 
 pub fn errno() -> i32 {