Make rust-intrinsic ABI unwindable

Stick `#[rustc_nounwind]` to all except `const_eval_select` to undo the
change for all other intrinsics.

Author: nbdd0121

core/src/ffi/mod.rs

@@ -615,12 +615,15 @@ impl<'f> Drop for VaListImpl<'f> {
 extern "rust-intrinsic" {
     /// Destroy the arglist `ap` after initialization with `va_start` or
     /// `va_copy`.
+    #[rustc_nounwind]
     fn va_end(ap: &mut VaListImpl<'_>);
 
     /// Copies the current location of arglist `src` to the arglist `dst`.
+    #[rustc_nounwind]
     fn va_copy<'f>(dest: *mut VaListImpl<'f>, src: &VaListImpl<'f>);
 
     /// Loads an argument of type `T` from the `va_list` `ap` and increment the
     /// argument `ap` points to.
+    #[rustc_nounwind]
     fn va_arg<T: sealed_trait::VaArgSafe>(ap: &mut VaListImpl<'_>) -> T;
 }