Auto merge of rust-lang#121516 - RalfJung:platform-intrinsics-begone, r=oli-obk

remove platform-intrinsics ABI; make SIMD intrinsics be regular intrinsics

`@Amanieu` `@workingjubilee` I don't think there is any reason these need to be "special"? The [original RFC](https://rust-lang.github.io/rfcs/1199-simd-infrastructure.html) indicated eventually making them stable, but I think that is no longer the plan, so seems to me like we can clean this up a bit.

Blocked on rust-lang/stdarch#1538, rust-lang#121542.

Author: bors

example/float-minmax-pass.rs

@@ -4,17 +4,14 @@
 
 // Test that the simd_f{min,max} intrinsics produce the correct results.
 
-#![feature(repr_simd, platform_intrinsics)]
+#![feature(repr_simd, core_intrinsics)]
 #![allow(non_camel_case_types)]
 
 #[repr(simd)]
 #[derive(Copy, Clone, PartialEq, Debug)]
 struct f32x4(pub f32, pub f32, pub f32, pub f32);
 
-extern "platform-intrinsic" {
-    fn simd_fmin<T>(x: T, y: T) -> T;
-    fn simd_fmax<T>(x: T, y: T) -> T;
-}
+use std::intrinsics::simd::*;
 
 fn main() {
     let x = f32x4(1.0, 2.0, 3.0, 4.0);