Rollup merge of #102232 - Urgau:stabilize-bench_black_box, r=TaKO8Ki

Stabilize bench_black_box

This PR stabilize `feature(bench_black_box)`.

```rust
pub fn black_box<T>(dummy: T) -> T;
```

The FCP was completed in https://github.com/rust-lang/rust/issues/64102.

`@rustbot` label +T-libs-api -T-libs

library/std/src/lib.rs

@@ -348,7 +348,6 @@
 #![feature(trace_macros)]
 //
 // Only used in tests/benchmarks:
-#![feature(bench_black_box)]
 //
 // Only for const-ness:
 #![feature(const_io_structs)]