tokio: fix remaining issues about atomic_u64_static_once_cell.rs (#5374)

Fixes #5373
Closes #5358 

- Add check for no_atomic_u64 & no_const_mutex_new (condition to atomic_u64_static_once_cell.rs is compiled)
- Allow unused_imports in TARGET_ATOMIC_U64_PROBE. I also tested other *_PROBE and found no other errors triggered by -D warning.
- Fix cfg of util::once_cell module

.github/workflows/ci.yml

@@ -300,6 +300,8 @@ jobs:
         with:
           toolchain: ${{ env.rust_nightly }}
           components: rust-src
+      - name: Install cargo-hack
+        uses: taiki-e/install-action@cargo-hack
       # Install linker and libraries for i686-unknown-linux-gnu
       - uses: taiki-e/setup-cross-toolchain-action@v1
         with:
@@ -307,6 +309,14 @@ jobs:
       - run: cargo test -Zbuild-std --target target-specs/i686-unknown-linux-gnu.json -p tokio --all-features
         env:
           RUSTFLAGS: --cfg tokio_unstable -Dwarnings --cfg tokio_no_atomic_u64
+      # https://github.com/tokio-rs/tokio/pull/5356
+      # https://github.com/tokio-rs/tokio/issues/5373
+      - run: cargo hack build -p tokio --feature-powerset --depth 2 -Z avoid-dev-deps --keep-going
+        env:
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings --cfg tokio_no_atomic_u64 --cfg tokio_no_const_mutex_new
+      - run: cargo hack build -p tokio --feature-powerset --depth 2 -Z avoid-dev-deps --keep-going
+        env:
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings --cfg tokio_no_atomic_u64
 
   features:
     name: features