Rollup merge of rust-lang#90621 - adamgemmell:dev/stabilise-target-feature, r=Amanieu

Stabilise `aarch64_target_feature`

This PR stabilises `aarch64_target_feature` - see rust-lang#90620

Author: matthiaskrgr

core/src/lib.rs

@@ -206,7 +206,7 @@
 #![feature(asm_const)]
 //
 // Target features:
-#![feature(aarch64_target_feature)]
+#![cfg_attr(bootstrap, feature(aarch64_target_feature))]
 #![feature(adx_target_feature)]
 #![feature(arm_target_feature)]
 #![feature(avx512_target_feature)]