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

src/test/rustdoc-ui/wasm-safe.rs

@@ -1,7 +1,5 @@
 // check-pass
 
-#![feature(wasm_target_feature)]
-
 #[cfg(any(target_arch = "wasm32", doc))]
 #[target_feature(enable = "simd128")]
 pub fn foo() {}

src/test/ui/simd/wasm-simd-indirect.rs

@@ -1,7 +1,5 @@
 // build-pass
 
-#![cfg_attr(target_arch = "wasm32", feature(wasm_simd, wasm_target_feature))]
-
 #[cfg(target_arch = "wasm32")]
 fn main() {
     unsafe {