Auto merge of #132458 - RalfJung:rustc-const-unstable, r=Amanieu

get rid of a whole bunch of unnecessary rustc_const_unstable attributes

In general, when a `const fn` is still unstable, it doesn't need a `#[rustc_const_unstable]` attribute. The only exception is functions that internally use things that can't be used in stable const fn yet.

So this gets rid of a whole bunch of `#[rustc_const_unstable]` in libcore.

Author: bors