Rollup merge of #125311 - calebzulawski:repr-packed-simd-intrinsics, r=workingjubilee

Make repr(packed) vectors work with SIMD intrinsics

In #117116 I fixed `#[repr(packed, simd)]` by doing the expected thing and removing padding from the layout.  This should be the last step in providing a solution to rust-lang/portable-simd#319

Author: workingjubilee