Fixup some stability annotations and feature directives

Author: alexcrichton

crates/core_arch/src/lib.rs

@@ -17,6 +17,7 @@
     no_core,
     rustc_attrs,
     stdsimd,
+    wasm_simd,
     staged_api,
     doc_cfg,
     mmx_target_feature,

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,