Auto merge of #106774 - Nugine:master, r=Amanieu

Stabilize cmpxchg16b_target_feature

Tracking issue for target features
+ #44839

stdarch issue
+ rust-lang/stdarch#827

stdarch PR
+ rust-lang/stdarch#1358

reference PR
+ rust-lang/reference#1331

It's my first time contributing to rust-lang/rust. Please tell me if I missed something.

core/src/lib.rs

@@ -244,7 +244,6 @@
 // Target features:
 #![feature(arm_target_feature)]
 #![feature(avx512_target_feature)]
-#![feature(cmpxchg16b_target_feature)]
 #![feature(hexagon_target_feature)]
 #![feature(mips_target_feature)]
 #![feature(powerpc_target_feature)]
@@ -253,6 +252,7 @@
 #![feature(sse4a_target_feature)]
 #![feature(tbm_target_feature)]
 #![feature(wasm_target_feature)]
+#![cfg_attr(bootstrap, feature(cmpxchg16b_target_feature))]
 
 // allow using `core::` in intra-doc links
 #[allow(unused_extern_crates)]