Properly define va_arg and va_list for aarch64-apple-darwin

From [Apple][]:

> Because of these changes, the type `va_list` is an alias for `char*`,
> and not for the struct type in the generic procedure call standard.

With this change `/x.py test --stage 1 src/test/ui/abi/variadic-ffi`
passes.

Fixes #78092

[Apple]: https://developer.apple.com/documentation/xcode/writing_arm64_code_for_apple_platforms

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -184,8 +184,8 @@ pub(super) fn emit_va_arg(
         "aarch64" if target.options.is_like_windows => {
             emit_ptr_va_arg(bx, addr, target_ty, false, Align::from_bytes(8).unwrap(), false)
         }
-        // iOS AArch64
-        "aarch64" if target.target_os == "ios" => {
+        // macOS / iOS AArch64
+        "aarch64" if target.options.is_like_osx => {
             emit_ptr_va_arg(bx, addr, target_ty, false, Align::from_bytes(8).unwrap(), true)
         }
         "aarch64" => emit_aapcs_va_arg(bx, addr, target_ty),