Rollup merge of #128150 - BoxyUwU:std_only_sized_const_params, r=workingjubilee

Stop using `unsized_const_parameters` in core/std

`feature(unsized_const_parameters)` is an incomplete feature and should not be used by core/std as it makes it can make it significantly harder to evolve the feature. It also just generally opens the possibility of introducing bugs on stable through std's backdoor.

The only usage of this feature in std is the `simd_shuffle_intrinsic` added in #119213. It doesn't seem to be used anywhere as far as I can tell so it is removed in this PR. All tests and codegen logic etc have been kept however.

r? `@workingjubilee`

Author: matthiaskrgr

tests/pass/intrinsics/portable-simd.rs

@@ -1,10 +1,23 @@
 //@compile-flags: -Zmiri-strict-provenance
-#![feature(portable_simd, adt_const_params, core_intrinsics, repr_simd)]
+#![feature(
+    portable_simd,
+    unsized_const_params,
+    adt_const_params,
+    rustc_attrs,
+    intrinsics,
+    core_intrinsics,
+    repr_simd
+)]
 #![allow(incomplete_features, internal_features)]
 use std::intrinsics::simd as intrinsics;
 use std::ptr;
 use std::simd::{prelude::*, StdFloat};
 
+extern "rust-intrinsic" {
+    #[rustc_nounwind]
+    pub fn simd_shuffle_generic<T, U, const IDX: &'static [u32]>(x: T, y: T) -> U;
+}
+
 fn simd_ops_f32() {
     let a = f32x4::splat(10.0);
     let b = f32x4::from_array([1.0, 2.0, 3.0, -4.0]);