wasm: Register the relaxed-simd target feature

This WebAssembly proposal is likely to reach stage 4 soon so this starts
the support in Rust for the proposal by adding a target feature that can
be enabled via attributes for the stdarch project to bind the
intrinsics.

Author: alexcrichton

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -286,6 +286,7 @@ const WASM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("mutable-globals", Some(sym::wasm_target_feature)),
     ("nontrapping-fptoint", Some(sym::wasm_target_feature)),
     ("reference-types", Some(sym::wasm_target_feature)),
+    ("relaxed-simd", Some(sym::wasm_target_feature)),
     ("sign-ext", Some(sym::wasm_target_feature)),
     ("simd128", None),
     // tidy-alphabetical-end