Add #![allow(internal_unstable)]

This is required to avoid build failures when using rustc features
intended only for use by the standard library.

library/stdarch/crates/core_arch/src/lib.rs

@@ -2,6 +2,7 @@
 #![allow(improper_ctypes_definitions)]
 #![allow(dead_code)]
 #![allow(unused_features)]
+#![allow(internal_features)]
 #![deny(rust_2018_idioms)]
 #![feature(
     custom_inner_attributes,

library/stdarch/crates/std_detect/src/lib.rs

@@ -19,8 +19,6 @@
 #![deny(clippy::missing_inline_in_public_items)]
 #![cfg_attr(test, allow(unused_imports))]
 #![no_std]
-// FIXME(Nilstrieb): Remove this once the compiler in stdarch CI has the internal_features lint.
-#![allow(unknown_lints)]
 #![allow(internal_features)]
 
 #[cfg(test)]