Feature gate #[simd]

Fixes #11721

src/libstd/lib.rs

@@ -52,13 +52,14 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://static.rust-lang.org/doc/master")];
 
-#[feature(macro_rules, globs, asm, managed_boxes, thread_local, link_args)];
+#[feature(macro_rules, globs, asm, managed_boxes, thread_local, link_args, simd)];
 
 // Don't link to std. We are std.
 #[no_std];
 
 #[deny(non_camel_case_types)];
 #[deny(missing_doc)];
+#[allow(unknown_features)];
 
 // When testing libstd, bring in libuv as the I/O backend so tests can print
 // things and all of the std::io tests have an I/O interface to run on top