Stabilize const_raw_ptr_deref for *const T

This stabilizes dereferencing immutable raw pointers in const contexts.
It does not stabilize `*mut T` dereferencing. This is placed behind the
`const_raw_mut_ptr_deref` feature gate.

library/core/src/lib.rs

@@ -156,7 +156,7 @@
 #![feature(const_impl_trait)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]
-#![feature(const_raw_ptr_deref)]
+#![cfg_attr(bootstrap, feature(const_raw_ptr_deref))]
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]