Re-add #![feature(target_feature_11)]

We removed this in 307c42f, but the
stabilization was reverted in rust-lang/rust#108654.

Author: taiki-e

crates/core_arch/src/lib.rs

@@ -30,6 +30,7 @@
     allow_internal_unstable,
     decl_macro,
     asm_const,
+    target_feature_11,
     inline_const,
     generic_arg_infer
 )]