Implement interrupt status register

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

sw/tests/test_main.c

@@ -2,6 +2,7 @@
 
 #include "sevenseg.h"
 #include "uart.h"
+#include "interrupt.h"
 
 int main(void)
 {
@@ -75,5 +76,14 @@ int main(void)
     printf("Done!\n\n");
 
 
+    printf("Testing irq_get_status...\n");
+    *(uint8_t*)0x7fff = 0xa5;
+    if (irq_get_status() != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+
     return retval != 0;
 }