Rollup merge of rust-lang#48887 - alexcrichton:update-stdsimd, r=kennytm

Update stdsimd module

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

Author: Mark-Simulacrum

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)]