Implement interrupt status register

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

sw/interrupt.h

@@ -0,0 +1,13 @@
+#ifndef _INTERRUPT_H
+#define _INTERRUPT_H
+
+#include <stdint.h>
+
+#define BUTTON  (1 << 0)
+
+void irq_int();
+void nmi_int();
+
+uint8_t irq_get_status();
+
+#endif