replace asm volatile("pause") with function

arch/x86_64/serial.c

@@ -1,6 +1,7 @@
 #include <zinnia/console.h>
 #include <zinnia/init.h>
 #include <zinnia/initgraph.h>
+#include <arch/cpu.h>
 #include <stdint.h>
 #include <x86_64/asm.h>
 
@@ -13,7 +14,7 @@ static bool is_ready() {
 static void serial_write(struct console* con, const char* buf, size_t count) {
     for (size_t i = 0; i < count; i++) {
         while (!is_ready())
-            asm volatile("pause");
+            arch_cpu_pause();
 
         asm_outb(COM1_BASE, buf[i]);
     }