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

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

Fixes: #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````

library/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)]