Stabilize bench_black_box

Author: Urgau

tests/fail/invalid_bool.rs

@@ -1,7 +1,7 @@
 // Validation makes this fail in the wrong place
 // Make sure we find these even with many checks disabled.
 //@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
-#![feature(bench_black_box)]
+
 
 fn main() {
     let b = unsafe { std::mem::transmute::<u8, bool>(2) };

tests/pass/float.rs

@@ -1,4 +1,4 @@
-#![feature(stmt_expr_attributes, bench_black_box)]
+#![feature(stmt_expr_attributes)]
 #![allow(arithmetic_overflow)]
 use std::fmt::Debug;
 use std::hint::black_box;

tests/pass/u128.rs

@@ -1,4 +1,3 @@
-#![feature(bench_black_box)]
 use std::hint::black_box as b;
 
 fn main() {