Rollup merge of rust-lang#52759 - stjepang:impl-send-sync-for-joinhandle, r=TimNN

Impl Send & Sync for JoinHandle

This is just a cosmetic change - it slightly relaxes and clarifies the public API without effectively promising any new guarantees.

Currently we have [these auto trait implementations](https://doc.rust-lang.org/nightly/std/thread/struct.JoinHandle.html#synthetic-implementations):

```rust
impl<T: Send> Send for JoinHandle<T> {}
impl<T: Sync> Sync for JoinHandle<T> {}
```

Bound `T: Send` doesn't make much sense because `JoinHandle<T>` can be created only when `T: Send`. Note that [`JoinHandle::<T>::join`](https://doc.rust-lang.org/nightly/std/thread/struct.JoinHandle.html#method.join) doesn't require `T: Send` so why should the `Send` impl?

And the `Sync` impl doesn't need `T: Sync` because `JoinHandle<T>` cannot even share `T` - it can only send it to the thread that calls `join`.

Author: Mark-Simulacrum

src/libstd/thread/mod.rs

@@ -1276,6 +1276,11 @@ impl<T> JoinInner<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct JoinHandle<T>(JoinInner<T>);
 
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
+unsafe impl<T> Send for JoinHandle<T> {}
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
+unsafe impl<T> Sync for JoinHandle<T> {}
+
 impl<T> JoinHandle<T> {
     /// Extracts a handle to the underlying thread.
     ///