Rollup merge of rust-lang#84871 - richkadel:no-coverage-unstable-only, r=nagisa

Disallows `#![feature(no_coverage)]` on stable and beta (using standard crate-level gating)

Fixes: rust-lang#84836

Removes the function-level feature gating solution originally implemented, and solves the same problem using `allow_internal_unstable`, so normal crate-level feature gating mechanism can still be used (which disallows the feature on stable and beta).

I tested this, building the compiler with and without `CFG_DISABLE_UNSTABLE_FEATURES=1`

With unstable features disabled, I get the expected result as shown here:

```shell
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc     src/test/run-make-fulldeps/coverage/no_cov_crate.rs
error[E0554]: `#![feature]` may not be used on the dev release channel
 --> src/test/run-make-fulldeps/coverage/no_cov_crate.rs:2:1
  |
2 | #![feature(no_coverage)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
```

r? ````@Mark-Simulacrum````
cc: ````@tmandry```` ````@wesleywiser````

Author: Dylan-DPC

core/src/cmp.rs

@@ -274,8 +274,7 @@ pub trait Eq: PartialEq<Self> {
     //
     // This should never be implemented by hand.
     #[doc(hidden)]
-    #[cfg_attr(not(bootstrap), feature(no_coverage))]
-    #[cfg_attr(not(bootstrap), no_coverage)]
+    #[cfg_attr(not(bootstrap), no_coverage)] // rust-lang/rust#84605
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn assert_receiver_is_total_eq(&self) {}
@@ -284,7 +283,7 @@ pub trait Eq: PartialEq<Self> {
 /// Derive macro generating an impl of the trait `Eq`.
 #[rustc_builtin_macro]
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow_internal_unstable(core_intrinsics, derive_eq, structural_match)]
+#[allow_internal_unstable(core_intrinsics, derive_eq, structural_match, no_coverage)]
 pub macro Eq($item:item) {
     /* compiler built-in */
 }

core/src/lib.rs

@@ -166,6 +166,7 @@
 #![feature(const_caller_location)]
 #![feature(slice_ptr_get)]
 #![feature(no_niche)] // rust-lang/rust#68303
+#![cfg_attr(not(bootstrap), feature(no_coverage))] // rust-lang/rust#84605
 #![feature(int_error_matching)]
 #![deny(unsafe_op_in_unsafe_fn)]