Rollup merge of #119590 - ChrisDenton:cfg-target-abi, r=Nilstrieb

Stabilize `cfg_target_abi`

This stabilizes the `cfg` option called `target_abi`:

```rust
#[cfg(target_abi = "eabihf")]
```

Tracking issue: #80970

fixes #78791
resolves #80970

compiler/rustc_feature/src/builtin_attrs.rs

@@ -25,7 +25,6 @@ pub type GatedCfg = (Symbol, Symbol, GateFn);
 const GATED_CFGS: &[GatedCfg] = &[
     // (name in cfg, feature, function to check if the feature is enabled)
     (sym::overflow_checks, sym::cfg_overflow_checks, cfg_fn!(cfg_overflow_checks)),
-    (sym::target_abi, sym::cfg_target_abi, cfg_fn!(cfg_target_abi)),
     (sym::target_thread_local, sym::cfg_target_thread_local, cfg_fn!(cfg_target_thread_local)),
     (
         sym::target_has_atomic_equal_alignment,