Implement interrupt status register

Upon an interrupt, you can read from the interrupt status register to
see what caused the interrupt.

sw/irq.c

@@ -1,7 +1,19 @@
 
+#include <stdint.h>
+
+#include "interrupt.h"
+
 // This is defined in main.c
 void puts(const char* s);
 
 void handle_irq() {
+    uint8_t status;
+
     puts("Interrupt Detected!\n");
+
+    status = irq_get_status();
+
+    if (status & BUTTON) {
+        puts("Button Interrupt!\n");
+    }
 }