Fixup some stability annotations and feature directives

Author: alexcrichton

crates/core_arch/src/lib.rs

@@ -38,6 +38,7 @@
     decl_macro
 )]
 #![cfg_attr(test, feature(test, abi_vectorcall, untagged_unions))]
+#![cfg_attr(all(test, target_arch = "wasm32"), feature(wasm_simd))]
 #![deny(clippy::missing_inline_in_public_items)]
 #![allow(
     clippy::inline_always,

crates/core_arch/tests/xcrate-macros.rs

@@ -1,4 +1,5 @@
-#![feature(stdsimd, wasm_simd)]
+#![feature(stdsimd)]
+#![cfg_attr(target_arch = "wasm32", feature(wasm_simd))]
 
 #[test]
 #[cfg(target_arch = "wasm32")]

examples/hex.rs

@@ -14,6 +14,7 @@
 
 #![feature(stdsimd, wasm_target_feature)]
 #![cfg_attr(test, feature(test))]
+#![cfg_attr(target_arch = "wasm32", feature(wasm_simd))]
 #![allow(
     clippy::result_unwrap_used,
     clippy::print_stdout,