chore(ci): Catch naive use of AtomicU64 early

AtomicU64 is not portable and use of it, without `target_has_atomic`,
will fail CI for rust-lang/rust when the cargo submodule gets updated
(see rust-lang#12981).

If there is a reason to use it, we could always `allow` in that one
case.

clippy.toml

@@ -6,3 +6,6 @@ disallowed-methods = [
     { path = "std::env::vars", reason = "not recommended to use in Cargo. See rust-lang/cargo#11588" },
     { path = "std::env::vars_os", reason = "not recommended to use in Cargo. See rust-lang/cargo#11588" },
 ]
+disallowed-types = [
+    "std::sync::atomic::AtomicU64",
+]