aix: error on unsupported target

Add `compile_error` macro invocation when declaring
architecture-specific definitions. This should ensure any new target
either gets proper review of the required types or otherwise provides
its target-specific definitions and bindings.

Author: dybucc

src/unix/aix/mod.rs

@@ -3344,5 +3344,7 @@ cfg_if! {
     if #[cfg(target_arch = "powerpc64")] {
         mod powerpc64;
         pub use self::powerpc64::*;
+    } else {
+        core::compile_error!("unsupported target");
     }
 }