Rollup merge of #66822 - RalfJung:miri-panic, r=oli-obk

libunwind_panic: adjust miri panic hack

We adjust the Miri hack in libpanic_unwind such that even with `cfg(miri)`, we build a version of libpanic_unwind that actually works.

This is needed to resolve https://github.com/integer32llc/rust-playground/issues/548.

r? @oli-obk @alexcrichton

src/libcore/intrinsics.rs

@@ -1348,9 +1348,11 @@ extern "rust-intrinsic" {
     pub fn ptr_offset_from<T>(ptr: *const T, base: *const T) -> isize;
 
     /// Internal hook used by Miri to implement unwinding.
+    /// Compiles to a NOP during non-Miri codegen.
+    ///
     /// Perma-unstable: do not use
     #[cfg(not(bootstrap))]
-    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> !;
+    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> ();
 }
 
 // Some functions are defined here because they accidentally got made