fix s3 a20

stages/s3/a20.s

@@ -1,12 +1,14 @@
+[bits 16]
+
 %include "fn.s"
 %include "ps2.s"
 
-
 %macro mov_out 3
   mov %1, %3
   out %2, %1
 %endmacro
 
+section .s3_text
 
 ; Check whether the A20 line is enabled. Writes to the boot sector identifier.
 ; Arguments: none