Rollup merge of #154248 - lms0806:issue_154034, r=TaKO8Ki

resolve :  mark repr_simd as internal

I changed ```repr_simd``` to ```internal``` and changed the position to ```feature-group-start: internal feature gates```.

close rust-lang/rust#154034

library/alloctests/benches/lib.rs

@@ -1,5 +1,6 @@
 // This is marked as `test = true` and hence picked up by `./x miri`, but that would be too slow.
 #![cfg(not(miri))]
+#![allow(internal_features)]
 #![feature(iter_next_chunk)]
 #![feature(repr_simd)]
 #![feature(slice_partition_dedup)]