Conditionaly compile based on architecture

This is in reference to issue PacktPublishing#5
I have also removed a redundant guard per advice from clippy

Author: Steve McFarlin

ch04/a-epoll/src/ffi.rs

@@ -11,13 +11,23 @@ extern "C" {
 }
 
 #[derive(Debug)]
+#[cfg(not(target_arch = "aarch64"))]
 #[repr(C, packed)]
 pub struct Event {
     pub(crate) events: u32,
     // Token to identify event
     pub(crate) epoll_data: usize,
 }
 
+#[derive(Debug)]
+#[cfg(target_arch = "aarch64")]
+#[repr(C)]
+pub struct Event {
+    pub(crate) events: u32,
+    // Token to identify event
+    pub(crate) epoll_data: usize,
+}
+
 impl Event {
     pub fn token(&self) -> usize {
         self.epoll_data

ch04/a-epoll/src/main.rs

@@ -49,7 +49,7 @@ fn handle_events(
 
         loop {
             match streams[index].read(&mut data) {
-                Ok(n) if n == 0 => {
+                Ok(0) => {
                     // FIX #4
                     // `insert` returns false if the value already existed in the set. We
                     // handle it here since we must be sure that the TcpStream is fully