Add default trait implementations for "c-unwind" ABI function pointers

Following up on #92964, only add default trait implementations for the
`c-unwind` family of function pointers. The previous attempt in #92964
added trait implementations for many more ABIs and ran into concerns
regarding the increase in size of the libcore rlib.

An attempt to abstract away function pointer types behind a unified
trait to reduce the duplication of trait impls is being discussed in #99531
but this change looks to be blocked on a lang MCP.

Following @RalfJung's suggestion in
rust-lang/rust#99531 (comment),
this commit is another cut at #92964 but it _only_ adds the impls for
`extern "C-unwind" fn` and `unsafe extern "C-unwind" fn`.

I am interested in landing this patch to unblock the stabilization of
the `c_unwind` feature.

RFC: rust-lang/rfcs#2945
Tracking Issue: rust-lang/rust#74990

core/src/lib.rs

@@ -174,6 +174,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(associated_type_bounds)]
 #![feature(auto_traits)]
+#![feature(c_unwind)]
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_has_atomic_equal_alignment)]

core/src/ptr/mod.rs

@@ -1938,16 +1938,22 @@ macro_rules! fnptr_impls_args {
  fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
  fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
  fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
+ fnptr_impls_safety_abi! { extern "C-unwind" fn($($Arg),+) -> Ret, $($Arg),+ }
+ fnptr_impls_safety_abi! { extern "C-unwind" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
  fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
  fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
  fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
+ fnptr_impls_safety_abi! { unsafe extern "C-unwind" fn($($Arg),+) -> Ret, $($Arg),+ }
+ fnptr_impls_safety_abi! { unsafe extern "C-unwind" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
  };
  () => {
  // No variadic functions with 0 parameters
  fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
  fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
+ fnptr_impls_safety_abi! { extern "C-unwind" fn() -> Ret, }
  fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
  fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
+ fnptr_impls_safety_abi! { unsafe extern "C-unwind" fn() -> Ret, }
  };
 }