cpufeatures: fix macOS build (#1066)

The "sm4" token wasn't being handled. To fix the bleeding, this adds it
but hardcoded to disabled.

See build failure here: https://github.com/RustCrypto/utils/actions/runs/8818083683/job/24206057206?pr=1065

cpufeatures/src/aarch64.rs

@@ -117,6 +117,9 @@ macro_rules! check {
                 && $crate::aarch64::sysctlbyname(b"hw.optional.armv8_2_sha3\0")
         }
     };
+    ("sm4") => {
+        false
+    };
 }
 
 /// Apple helper function for calling `sysctlbyname`.