cpu/arm: allow dead code in cpu/arm mod when building on non-arm

Other architectures may not need to do feature checks, and therefore
Feature::available, Feature::mask, etc are never used/read.

This snippet mirrors the similar bit at the top of cpu/intel.

Signed-off-by: Eric Richter <erichte@linux.ibm.com>

src/cpu/arm.rs

@@ -12,6 +12,11 @@
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
+#![cfg_attr(
+    not(any(target_arch = "aarch64", target_arch = "arm")),
+    allow(dead_code)
+)]
+
 #[cfg(all(
     any(target_os = "android", target_os = "linux"),
     any(target_arch = "aarch64", target_arch = "arm")