Rollup merge of #132730 - joboet:after_main_sync, r=Noratrieb

std: allow after-main use of synchronization primitives

By creating an unnamed thread handle when the actual one has already been destroyed, synchronization primitives using thread parking can be used even outside the Rust runtime.

This also fixes an inefficiency in the queue-based `RwLock`: if `thread::current` was not initialized yet, it will create a new handle on every parking attempt without initializing `thread::current`. The private `current_or_unnamed` function introduced here fixes this.

library/std/src/lib.rs

@@ -174,9 +174,6 @@
 //!
 //! - after-main use of thread-locals, which also affects additional features:
 //!   - [`thread::current()`]
-//!   - [`thread::scope()`]
-//!   - [`sync::mpmc`]
-//!   - [`sync::mpsc`]
 //! - before-main stdio file descriptors are not guaranteed to be open on unix platforms
 //!
 //!