move gdt, use fixed addr

stage_2/main.s

@@ -39,7 +39,7 @@ s2_main:
   ; Copy the GDT
   mov cx, GDT_FLAT_LEN
   mov si, gdt_flat
-  mov di, GDT_ADDR
+  mov di, GDT_FLAT_ADDR
   rep movsb
 
   ; Ensure interrupts are definitely disabled.
@@ -86,9 +86,7 @@ section .s3_data
 
 gdt_flat_slice:
   dw GDT_FLAT_LEN
-  dd gdt_flat
-
-global gdt_flat_slice
+  dd GDT_FLAT_ADDR
 
 ; Segment descriptor layout
 ; | Range (bits) | Field         |