GDT finally works

kernel/hal/x86_64/hal.c

@@ -0,0 +1,21 @@
+#include <stdint.h>
+#include <stdbool.h>
+#include "hal/hal.h"
+#include "kprintf.h"
+#include "serial.h"
+#include "gdt.h"
+
+void hal_init(void) {
+  if (!serial_init()) {
+    hal_hang(); // going further makes no sense
+  }
+  LOG("hal", "serial init\n");
+  gdt_init();
+}
+
+__attribute__((noreturn)) void hal_hang(void) {
+  for(;;) {
+    asm("hlt");
+  }
+}
+