Update library functions with stability attributes

This may not be strictly minimal, but all unstable functions also need a
`rustc_const_unstable` attribute.

library/std/src/lib.rs

@@ -237,6 +237,7 @@
 #![feature(concat_idents)]
 #![feature(const_cstr_unchecked)]
 #![feature(const_fn_transmute)]
+#![feature(const_fn)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]
 #![feature(const_ipv4)]
@@ -306,6 +307,7 @@
 #![feature(str_internals)]
 #![feature(test)]
 #![feature(thread_local)]
+#![feature(thread_local_internals)]
 #![feature(toowned_clone_into)]
 #![feature(total_cmp)]
 #![feature(trace_macros)]