Propogate the task attributes to the spawn handles

This allows tasks to be gated by `cfg` attributes when also using
monotonics. For example:

```rust
#[cfg(feature = "logging")]
#[task(shared = [logger])]
fn logger_init(mut cx: logger_init::Context) {
    /* ... */
}
```

Without this change, the reschedule_at() implementation is
unconditionally included even though it references the SpawnHandle from
its task module, which is _conditionally_ included. This resulted in
compiler errors like the following:

```
error[E0433]: failed to resolve: use of undeclared crate or module `logger_init`
   --> src/main.rs:243:8
    |
243 |     fn logger_init(mut cx: logger_init::Context) {
    |        ^^^^^^^^^^^ use of undeclared crate or module `logger_init`
```

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