Allow internal_features lint when building with "unstable"

Author: Amanieu

compiler-builtins/libm/src/lib.rs

@@ -1,7 +1,8 @@
 //! libm in pure Rust
 #![deny(warnings)]
 #![no_std]
-#![cfg_attr(all(feature = "unstable"), feature(core_intrinsics))]
+#![cfg_attr(feature = "unstable", allow(internal_features))]
+#![cfg_attr(feature = "unstable", feature(core_intrinsics))]
 #![allow(clippy::unreadable_literal)]
 #![allow(clippy::many_single_char_names)]
 #![allow(clippy::needless_return)]