Rollup merge of #136948 - workingjubilee:split-off-extern-system-varargs, r=compiler-errors

Split out the `extern_system_varargs` feature

After the stabilization PR was opened, `extern "system"` functions were added to `extended_varargs_abi_support`. This has a number of questions regarding it that were not discussed and were somewhat surprising. It deserves to be considered as its own feature, separate from `extended_varargs_abi_support`.

Tracking issue:
- https://github.com/rust-lang/rust/issues/136946

compiler/rustc_span/src/symbol.rs

@@ -881,6 +881,7 @@ symbols! {
         extern_crate_self,
         extern_in_paths,
         extern_prelude,
+        extern_system_varargs,
         extern_types,
         external,
         external_doc,