[libprefix] Fix building on 64-bit FreeBSD 8.4

Reported-by: Pavel Antonov <holly@istu.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

 #endif /* KEEP_IT_REAL */
 
 	/* Switch back to original stack and zero .bss16 */
-	lss	%ss:(%esp), %esp
+	addr32 lss %ss:(%esp), %esp
 	pushl	%edi
 	pushw	%es
 	movw	%bx, %es