Auto merge of #128771 - carbotaniuman:stabilize_unsafe_attr, r=nnethercote

Stabilize `unsafe_attributes`

# Stabilization report

## Summary

This is a tracking issue for the RFC 3325: unsafe attributes

We are stabilizing `#![feature(unsafe_attributes)]`,  which makes certain attributes considered 'unsafe', meaning that they must be surrounded by an `unsafe(...)`, as in `#[unsafe(no_mangle)]`.

RFC: rust-lang/rfcs#3325
Tracking issue: #123757

## What is stabilized

### Summary of stabilization

Certain attributes will now be designated as unsafe attributes, namely, `no_mangle`, `export_name`, and `link_section` (stable only), and these attributes will need to be called by surrounding them in `unsafe(...)` syntax. On editions prior to 2024, this is simply an edition lint, but it will become a hard error in 2024. This also works in `cfg_attr`, but `unsafe` is not allowed for any other attributes, including proc-macros ones.

```rust
#[unsafe(no_mangle)]
fn a() {}

#[cfg_attr(any(), unsafe(export_name = "c"))]
fn b() {}
```

For a table showing the attributes that were considered to be included in the list to require unsafe, and subsequent reasoning about why each such attribute was or was not included, see [this comment here](https://github.com/rust-lang/rust/pull/124214#issuecomment-2124753464)

## Tests

The relevant tests are in `tests/ui/rust-2024/unsafe-attributes` and `tests/ui/attributes/unsafe`.

compiler/rustc_feature/src/unstable.rs

@@ -622,8 +622,6 @@ declare_features! (
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
     /// Allows unnamed fields of struct and union type
     (incomplete, unnamed_fields, "1.74.0", Some(49804)),
-    /// Allows unsafe attributes.
-    (unstable, unsafe_attributes, "1.80.0", Some(123757)),
     /// Allows const generic parameters to be defined with types that
     /// are not `Sized`, e.g. `fn foo<const N: [u8]>() {`.
     (incomplete, unsized_const_params, "CURRENT_RUSTC_VERSION", Some(95174)),