fix alignment for chan_handle structs; rust equiv is translated

to char[16], not struct{long,long}

src/rt/rust_task.h

@@ -44,7 +44,7 @@ struct frame_glue_fns {
 // library. This struct must agree with the std::task::rust_task record.
 struct rust_task_user {
     rust_task_id id;
-    uint32_t notify_enabled;   // this is way more bits than necessary, but it
+    intptr_t notify_enabled;   // this is way more bits than necessary, but it
                                // simplifies the alignment.
     chan_handle notify_chan;
     uintptr_t rust_sp;         // Saved sp when not running.