x86_64: init local apic on boot, or legacy pic/pit as fallback

arch/x86_64/pit.c

@@ -0,0 +1,29 @@
+#include <socks/printk.h>
+#include <arch/ports.h>
+#include <arch/irq.h>
+
+static int pit_callback(void)
+{
+	printk("tick");
+	return 0;
+}
+
+static irq_hook_t pit_irq_hook = {
+	.irq_callback = pit_callback
+};
+
+void pit_init(unsigned int hz)
+{
+	unsigned int divisor = 1193180 / hz;
+
+	outportb(0x43, 0x36);
+	uint8_t lo = (uint8_t)(divisor & 0xFF);
+	uint8_t hi = (uint8_t)((divisor >> 8) & 0xFF);
+
+	outportb(0x40, lo);
+	outportb(0x40, hi);
+
+	hook_irq(IRQ0, &pit_irq_hook);
+
+	printk("clock: 8253 PIT initialised at %uHz", hz);
+}