x86_64: add temporary serial driver for printk

arch/x86_64/serial.c

@@ -0,0 +1,30 @@
+#include <arch/serial.h>
+#include <arch/ports.h>
+
+static int transmit_empty(int device)
+{
+    return inportb(device + 5) & 0x20;
+}
+
+void serial_send_byte(int device, char out)
+{
+    unsigned int _count = 0;
+    while (!transmit_empty(device)) {
+        _count++;
+    }
+
+    outportb(device, out);
+
+    while (!transmit_empty(device)) {
+        _count++;
+    }
+}
+
+void serial_putchar(int port, char ch)
+{
+    if (ch == '\n') {
+        serial_send_byte(port, '\r');
+    }
+
+    serial_send_byte(port, ch);
+}