Mark #[rustc_std_internal_symbol] as extern indicator

It currently implies #[no_mangle] which is alread an extern indicator,
but this will change in a future commit.

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -174,6 +174,7 @@ impl CodegenFnAttrs {
     /// * `#[linkage]` is present
     pub fn contains_extern_indicator(&self) -> bool {
         self.flags.contains(CodegenFnAttrFlags::NO_MANGLE)
+            || self.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
             || self.export_name.is_some()
             || match self.linkage {
                 // These are private, so make sure we don't try to consider