Enable varargs support for calling conventions other than C or cdecl

This patch makes it possible to use varargs for calling conventions,
which are either based on C (like efiapi) or C is based
on them (for example sysv64 and win64).

compiler/rustc_span/src/symbol.rs

@@ -694,6 +694,7 @@ symbols! {
         export_name,
         expr,
         extended_key_value_attributes,
+        extended_varargs_abi_support,
         extern_absolute_paths,
         extern_crate_item_prelude,
         extern_crate_self,