Auto merge of #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 https://github.com/rust-lang/stdarch/pull/1538, https://github.com/rust-lang/rust/pull/121542.

compiler/rustc_mir_transform/src/ffi_unwind_calls.rs

@@ -34,7 +34,6 @@ fn abi_can_unwind(abi: Abi) -> bool {
         | CCmseNonSecureCall
         | Wasm
         | RustIntrinsic
-        | PlatformIntrinsic
         | Unadjusted => false,
         Rust | RustCall | RustCold => true,
     }