Ignore buggy clippy::incompatible_msrv lint

This lint doesn't consider cfg.

```
error: current MSRV (Minimum Supported Rust Version) is `1.34.0` but this item is stable since `1.45.0`
     --> src\imp\core_atomic.rs:314:32
      |
  314 |                     self.inner.fetch_max(val, order)
      |                                ^^^^^^^^^^^^^^^^^^^^^
  ...
  415 | atomic_int!(AtomicIsize, isize);
      | ------------------------------- in this macro invocation
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
      = note: `-D clippy::incompatible-msrv` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(clippy::incompatible_msrv)]`
      = note: this error originates in the macro `atomic_int` (in Nightly builds, run with -Z macro-backtrace for more info)
```

.github/workflows/ci.yml

@@ -112,6 +112,7 @@ jobs:
             flags: -Z codegen-backend=cranelift
           - rust: nightly-2024-02-05 # TODO: core_simd bug https://github.com/rust-lang/portable-simd/pull/396
             target: aarch64_be-unknown-linux-gnu
+            flags: -A unknown_lints # TODO: buggy clippy::incompatible_msrv lint is not available on nightly-2024-02-05, so unknown_lints also need to be allowed.
           - rust: stable
             target: aarch64-unknown-linux-musl
           - rust: stable

bench/benches/bench.rs

@@ -5,6 +5,7 @@
     internal_features,
     unused_extern_crates,
     clippy::duplicate_mod,
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
     clippy::inline_always,
     clippy::naive_bytecount,
     clippy::only_used_in_recursion,

portable-atomic-util/src/lib.rs

@@ -58,6 +58,9 @@ See [#1] for other primitives being considered for addition to this crate.
     clippy::std_instead_of_alloc,
     clippy::std_instead_of_core,
 )]
+#![allow(
+    clippy::incompatible_msrv, // false positive: this lint doesn't consider cfg
+)]
 // docs.rs only
 #![cfg_attr(portable_atomic_doc_cfg, feature(doc_cfg))]
 

src/lib.rs

@@ -211,6 +211,7 @@ RUSTFLAGS="--cfg portable_atomic_no_outline_atomics" cargo ...
 #![cfg_attr(not(portable_atomic_no_asm), warn(missing_docs))] // module-level #![allow(missing_docs)] doesn't work for macros on old rustc
 #![allow(
     clippy::cast_lossless,
+    clippy::incompatible_msrv, // false positive: this lint doesn't consider cfg
     clippy::inline_always,
     clippy::naive_bytecount,
     clippy::unreadable_literal

tests/helper/src/lib.rs

@@ -3,6 +3,7 @@
 #![no_std]
 #![allow(
     missing_debug_implementations,
+    clippy::incompatible_msrv,
     clippy::missing_panics_doc,
     clippy::thread_local_initializer_can_be_made_const, // clippy bug: this lint doesn't consider MSRV
     clippy::undocumented_unsafe_blocks