Don't pull in a dependency on pthread_kill in single-threaded builds

Fixes a regression preventing single-threaded Emscripten builds from
linking successfully without passing `-pthread` to emcc.

Author: castholm

lib/std/Io/Threaded.zig

@@ -774,9 +774,10 @@ fn await(
     result_alignment: std.mem.Alignment,
 ) void {
     _ = result_alignment;
+    if (builtin.single_threaded) return;
     const t: *Threaded = @ptrCast(@alignCast(userdata));
-    const closure: *AsyncClosure = @ptrCast(@alignCast(any_future));
-    closure.waitAndFree(t.allocator, result);
+    const ac: *AsyncClosure = @ptrCast(@alignCast(any_future));
+    ac.waitAndFree(t.allocator, result);
 }
 
 fn cancel(
@@ -786,6 +787,7 @@ fn cancel(
     result_alignment: std.mem.Alignment,
 ) void {
     _ = result_alignment;
+    if (builtin.single_threaded) return;
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const ac: *AsyncClosure = @ptrCast(@alignCast(any_future));
     ac.closure.requestCancel();