replace impl of globa_async_handle with one using atomic compare-and-swap

src/rt/rust_kernel.cpp

@@ -27,10 +27,9 @@ rust_kernel::rust_kernel(rust_env *env) :
     // set up storage of pointers needed to
     // access the global loop.
     global_loop_chan = 0;
-    int foo = 0;
-    async_handle_stub = (void*)&foo;
-    global_async_handle = &async_handle_stub;
-    *global_async_handle = (void*)0;
+    async_handle_inner = (uintptr_t)0;
+    global_async_handle = &async_handle_inner;
+    *global_async_handle = (uintptr_t)0;
 
     // Create the single threaded scheduler that will run on the platform's
     // main thread