[refactor] Organize INT

kernel/serial.cpp

@@ -1,4 +1,5 @@
 #include <serial.h>
+#include <common.h>
 
 extern EFI_SYSTEM_TABLE *ST;
 
@@ -34,7 +35,7 @@ void serial_write(const char *str) {
 void serial_write_hex(UINTN val) {
     char buf[19];
     buf[0] = '0'; buf[1] = 'x';
-    for (int i = 17; i >= 2; i--) {
+    for (SSINT32 i = 17; i >= 2; i--) {
         UINTN digit = val & 0xF;
         buf[i] = digit < 10 ? '0' + digit : 'A' + digit - 10;
         val >>= 4;