Add instability attribute on private const_strlen function

A `rustc_const_stable` attribute by itself has nonintuitive purpose when
placed in a public module.

Separately, it would probably be okay to rename `const_strlen` to just
`strlen` to make it more clear this is our general-purpose
implementation of strlen now, not something specifically for const
(avoiding confusion like in PR 127444).

Author: dtolnay

library/core/src/ffi/c_str.rs

@@ -740,6 +740,7 @@ impl AsRef<CStr> for CStr {
 /// The pointer must point to a valid buffer that contains a NUL terminator. The NUL must be
 /// located within `isize::MAX` from `ptr`.
 #[inline]
+#[unstable(feature = "cstr_internals", issue = "none")]
 #[rustc_const_stable(feature = "const_cstr_from_ptr", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 const unsafe fn const_strlen(ptr: *const c_char) -> usize {