x86: Add the adx target feature to whitelist

Requested in rust-lang-nursery/stdsimd#322 this is hopefully the first
step!

src/librustc_codegen_llvm/llvm_util.rs

@@ -124,6 +124,7 @@ const AARCH64_WHITELIST: &[(&str, Option<&str>)] = &[
 ];
 
 const X86_WHITELIST: &[(&str, Option<&str>)] = &[
+    ("adx", Some("adx_target_feature")),
     ("aes", None),
     ("avx", None),
     ("avx2", None),