Allow internal use of stdsimd from detect_feature

This allows using feature detection macros, without placing a
requirement of enabled stdsimd feature gate from end users.

A follow-up to changes from #1311, which introduced the new macro.

crates/std_detect/src/detect/macros.rs

@@ -1,4 +1,5 @@
 #[macro_export]
+#[allow_internal_unstable(stdsimd)]
 macro_rules! detect_feature {
     ($feature:tt, $feature_lit:tt) => {
         $crate::detect_feature!($feature, $feature_lit : $feature_lit)