Multiple changes:

- Makefile: fix linker script path
- irq.c: `irqN()` stubs
- irq.h: fix missing header
- isr.h/isr.c extern `interrupt_handlers`
- utils.c: remove duplicate `memcmp`

kernel/isr.h

@@ -6,6 +6,7 @@
 #define MAX_INTERRUPTS 256
 
 typedef void (*isr_callback_t)(void);
+extern isr_callback_t interrupt_handlers[MAX_INTERRUPTS];
 
 void isr_handler(uint32_t int_num, uint32_t err_code);
 void register_interrupt_handler(uint8_t n, isr_callback_t handler);