cpu: add irq depth counting

Add a way of checking if a cpu is in the middle of handling an irq and
how deep it is into it. This will allow for future code to conditionally
run only in/outside interrupts.

kernel-src/include/x86-64/arch/cpu.h

@@ -30,6 +30,8 @@ typedef struct cpu_t {
 
 	uint64_t scratch; // expected to be here by other code
 
+	unsigned long irq_depth; // expected to be here by other code
+
 	timekeeper_source_t *timekeeper_source;
 	timekeeper_source_info_t *timekeeper_source_info;
 	time_t timekeeper_source_base_ticks;