std: Stabilize wasm simd intrinsics

This commit performs two changes to stabilize Rust support for
WebAssembly simd intrinsics:

* The stdarch submodule is updated to pull in rust-lang/stdarch#1179.
* The `wasm_target_feature` feature gate requirement for the `simd128`
  feature has been removed, stabilizing the name `simd128`.

This should conclude the FCP started on #74372 and...

Closes #74372

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -205,7 +205,7 @@ const RISCV_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
 ];
 
 const WASM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
-    ("simd128", Some(sym::wasm_target_feature)),
+    ("simd128", None),
     ("atomics", Some(sym::wasm_target_feature)),
     ("nontrapping-fptoint", Some(sym::wasm_target_feature)),
 ];