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

Following up on rust-lang#92964, only add default trait implementations for the
`c-unwind` family of function pointers. The previous attempt in rust-lang#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 rust-lang#99531
but this change looks to be blocked on a lang MCP.

Following @RalfJung's suggestion in
rust-lang#99531 (comment),
this commit is another cut at rust-lang#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#74990

Author: lopopolo

library/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)]

library/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, }
     };
 }