Auto merge of rust-lang#117468 - daxpedda:wasm-relaxed-simd, r=wesleywiser

Stabilize Wasm relaxed SIMD

This PR stabilizes [Wasm relaxed SIMD](https://github.com/WebAssembly/relaxed-simd) which has already reached [phase 4](https://github.com/WebAssembly/proposals/tree/04fa8c810e1dc99ab399e41052a6e427ee988180?tab=readme-ov-file#phase-4---standardize-the-feature-wg).

Tracking issue: rust-lang#111196
Implementation PR: rust-lang/stdarch#1393
Documentation: rust-lang/reference#1421
Stdarch: rust-lang/stdarch#1494

Closes rust-lang#111196.

Author: bors

compiler/rustc_target/src/target_features.rs

@@ -322,7 +322,7 @@ const WASM_ALLOWED_FEATURES: &[(&str, Stability)] = &[
     ("mutable-globals", Stable),
     ("nontrapping-fptoint", Stable),
     ("reference-types", Unstable(sym::wasm_target_feature)),
-    ("relaxed-simd", Unstable(sym::wasm_target_feature)),
+    ("relaxed-simd", Stable),
     ("sign-ext", Stable),
     ("simd128", Stable),
     // tidy-alphabetical-end

library/stdarch

@@ -0,0 +1,9 @@
+//@ only-wasm32-wasip1
+//@ compile-flags: -Ctarget-feature=+relaxed-simd --crate-type=lib
+//@ build-pass
+
+use std::arch::wasm32::*;
+
+pub fn test(a: v128, b: v128, m: v128) -> v128 {
+    i64x2_relaxed_laneselect(a, b, m)
+}