Update stdsimd module

Pulls in a redesigned `std::simd` module as well as a replacement for the
`is_target_feature_detected!` macro

src/libcore/lib.rs

@@ -74,7 +74,9 @@
 #![feature(concat_idents)]
 #![feature(const_fn)]
 #![feature(custom_attribute)]
+#![feature(doc_cfg)]
 #![feature(doc_spotlight)]
+#![feature(fn_must_use)]
 #![feature(fundamental)]
 #![feature(i128_type)]
 #![feature(inclusive_range_syntax)]

stdsimd @ ab9356f2

-Subproject commit 678cbd325c84070c9dbe4303969fbd2734c0b4ee
+Subproject commit ab9356f2af650815d339d77306f0d09c44d531ad