Provide a dummy NO_XBOX option that lives in opt_xbox.h for pc98.

This allows us to eliminate a three ifdef PC98 instances.

sys/conf/options.pc98

@@ -105,3 +105,7 @@ DEV_NPX			opt_npx.h
 # Debugging
 STOP_NMI		opt_cpu.h
 NPX_DEBUG		opt_npx.h
+
+# FreeBSD/i386 supports xbox and is shred with pc98 so we need to define dummy
+# opt_xbox.h
+NO_XBOX			opt_xbox.h

sys/i386/i386/pmap.c

@@ -106,9 +106,7 @@ __FBSDID("$FreeBSD$");
 #include "opt_cpu.h"
 #include "opt_pmap.h"
 #include "opt_msgbuf.h"
-#ifndef PC98 
 #include "opt_xbox.h"
-#endif
 
 #include <sys/param.h>
 #include <sys/systm.h>

sys/i386/i386/vm_machdep.c

@@ -47,9 +47,7 @@ __FBSDID("$FreeBSD$");
 #include "opt_npx.h"
 #include "opt_reset.h"
 #include "opt_cpu.h"
-#ifndef PC98 
 #include "opt_xbox.h"
-#endif
 
 #include <sys/param.h>
 #include <sys/systm.h>

sys/i386/pci/pci_cfgreg.c

@@ -30,9 +30,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#ifndef PC98
 #include "opt_xbox.h"
-#endif
 
 #include <sys/param.h>
 #include <sys/systm.h>