[ASM] Add OFF(x) macro for both GAS and ML

Same from `subsystems/mvdm/dos/asmxtras.inc`

Author: iLauncherDev

sdk/include/asm/asm.inc

@@ -172,6 +172,9 @@ ENDM
     .skip size, fill
 ENDM
 
+/* OFFset macro */
+#define OFF(x) offset x
+
 #define lXdtPrefix fword ptr
 
 ljmp MACRO segment, offset
@@ -324,6 +327,9 @@ ENDM
 #define REPEAT .rept
 #define ENDR .endr
 
+/* OFFset macro */
+#define OFF(x) x
+
 #define lXdtPrefix
 
 .macro ljmp segment, offset