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:🧵: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.

library/std/src/lib.rs

@@ -520,20 +520,20 @@ pub mod task {
     pub use alloc::task::*;
 }
 
-// Platform-abstraction modules
+// The runtime entry point and a few unstable public functions used by the
+// compiler
 #[macro_use]
-mod sys_common;
+pub mod rt;
+
+// Platform-abstraction modules
 mod sys;
+mod sys_common;
 
 pub mod alloc;
 
 // Private support modules
 mod panicking;
 
-// The runtime entry point and a few unstable public functions used by the
-// compiler
-pub mod rt;
-
 #[path = "../../backtrace/src/lib.rs"]
 #[allow(dead_code, unused_attributes)]
 mod backtrace_rs;