x86: don't build i8237.o

This complements c/s 155:fbfa306ab465 ("Suppress all use of ISA DMA on
Xen") - the code is simply useless on Xen.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/arch/i386/kernel/Makefile
+++ b/arch/i386/kernel/Makefile
@@ -84,5 +84,5 @@ $(obj)/vsyscall-syms.o: $(src)/vsyscall.
 early_printk-y            += ../../x86_64/kernel/early_printk.o
 k8-y                      += ../../x86_64/kernel/k8.o
 
-disabled-obj-$(CONFIG_XEN) := i8259.o reboot.o smpboot.o
+disabled-obj-$(CONFIG_XEN) := i8237.o i8259.o reboot.o smpboot.o
 %/head.o %/head.s: $(if $(CONFIG_XEN),EXTRA_AFLAGS,dummy) :=
--- a/arch/x86_64/kernel/Makefile
+++ b/arch/x86_64/kernel/Makefile
@@ -61,5 +61,5 @@ dmi_scan-y			+= ../../i386/kernel/dmi_sc
 time-$(CONFIG_XEN)		+= ../../i386/kernel/time.o
 pci-dma-$(CONFIG_XEN)		+= ../../i386/kernel/pci-dma.o
 
-disabled-obj-$(CONFIG_XEN)	:= i8259.o reboot.o smpboot.o trampoline.o
+disabled-obj-$(CONFIG_XEN)	:= i8237.o i8259.o reboot.o smpboot.o trampoline.o
 %/head.o %/head.s: $(if $(CONFIG_XEN),EXTRA_AFLAGS,dummy) :=