make arch an optional backend

crates/pacdef_core/src/backend/actual/mod.rs

@@ -1,2 +1,3 @@
+#[cfg(feature = "arch")]
 pub mod arch;
 pub mod rust;

crates/pacdef_core/src/backend/mod.rs

@@ -12,6 +12,7 @@ use pacdef_macros::Register;
 
 #[derive(Debug, Register)]
 pub enum Backends {
+    #[cfg(feature = "arch")]
     Arch,
     Rust,
 }