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`.

compiler/rustc_span/src/symbol.rs

@@ -879,6 +879,7 @@ symbols! {
         extern_crate_self,
         extern_in_paths,
         extern_prelude,
+        extern_system_varargs,
         extern_types,
         external,
         external_doc,