`multicore::Core::spawn`'s entrypoint should return `!` according to docs

from the docs:

> Spawn a function on this core.
> The closure should not return. It is currently defined as -> () because -> ! is not yet stable.

returning `!` has been stable for a while now -- the following code compiles under rustc 1.85.0:

```rust
fn core_entry() -> ! {
    loop {}
}
```