Clean up special function const checks

Mark them as const and `#[rustc_do_not_const_check]` instead of hard-coding
them in const-eval checks.

library/core/src/lib.rs

@@ -107,6 +107,7 @@
 #![feature(const_discriminant)]
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
+#![feature(const_fmt_arguments_new)]
 #![feature(const_heap)]
 #![feature(const_inherent_unchecked_arith)]
 #![feature(const_int_unchecked_arith)]