Auto merge of #123550 - GnomedDev:remove-initial-arc, r=Noratrieb

Remove the `Arc` rt::init allocation for thread info

Removes an allocation pre-main by just not storing anything in std:🧵:Thread for the main thread.
- The thread name can just be a hard coded literal, as was done in #123433.
- Storing ThreadId and Parker in a static that is initialized once at startup. This uses SyncUnsafeCell and MaybeUninit as this is quite performance critical and we don't need synchronization or to store a tag value and possibly leave in a panic.

library/std/src/lib.rs

@@ -364,6 +364,7 @@
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(strict_provenance_atomic_ptr)]
+#![feature(sync_unsafe_cell)]
 #![feature(ub_checks)]
 // tidy-alphabetical-end
 //