Auto merge of #89011 - bjorn3:restructure_rt, r=dtolnay

Auto merge of #89011 - bjorn3:restructure_rt, r=dtolnay

Restructure std::rt

These changes should reduce binary size slightly while at the same slightly improving performance of startup, thread spawning and `std::thread::current()`. I haven't verified if the compiler is able to optimize some of these cases already, but at least for some others the compiler is unable to do these optimizations as they slightly change behavior in cases where program startup would crash anyway by omitting a backtrace and panic location.

I can remove 6f6bb16 if preferred.