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_target/src/spec/abi/mod.rs

@@ -54,7 +54,6 @@ pub enum Abi {
     },
     RustIntrinsic,
     RustCall,
-    PlatformIntrinsic,
     Unadjusted,
     /// For things unlikely to be called, where reducing register pressure in
     /// `extern "Rust"` callers is worth paying extra cost in the callee.
@@ -129,7 +128,6 @@ const AbiDatas: &[AbiData] = &[
     AbiData { abi: Abi::System { unwind: true }, name: "system-unwind" },
     AbiData { abi: Abi::RustIntrinsic, name: "rust-intrinsic" },
     AbiData { abi: Abi::RustCall, name: "rust-call" },
-    AbiData { abi: Abi::PlatformIntrinsic, name: "platform-intrinsic" },
     AbiData { abi: Abi::Unadjusted, name: "unadjusted" },
     AbiData { abi: Abi::RustCold, name: "rust-cold" },
     AbiData { abi: Abi::RiscvInterruptM, name: "riscv-interrupt-m" },
@@ -199,10 +197,6 @@ pub fn is_stable(name: &str) -> Result<(), AbiDisabled> {
             feature: sym::intrinsics,
             explain: "intrinsics are subject to change",
         }),
-        "platform-intrinsic" => Err(AbiDisabled::Unstable {
-            feature: sym::platform_intrinsics,
-            explain: "platform intrinsics are experimental and possibly buggy",
-        }),
         "vectorcall" => Err(AbiDisabled::Unstable {
             feature: sym::abi_vectorcall,
             explain: "vectorcall is experimental and subject to change",
@@ -299,11 +293,10 @@ impl Abi {
             System { unwind: true } => 28,
             RustIntrinsic => 29,
             RustCall => 30,
-            PlatformIntrinsic => 31,
-            Unadjusted => 32,
-            RustCold => 33,
-            RiscvInterruptM => 34,
-            RiscvInterruptS => 35,
+            Unadjusted => 31,
+            RustCold => 32,
+            RiscvInterruptM => 33,
+            RiscvInterruptS => 34,
         };
         debug_assert!(
             AbiDatas

compiler/rustc_target/src/spec/mod.rs

@@ -2448,7 +2448,6 @@ impl Target {
             | System { .. }
             | RustIntrinsic
             | RustCall
-            | PlatformIntrinsic
             | Unadjusted
             | Cdecl { .. }
             | RustCold => true,