x86-64: tell sparse we're a 64-bit platform.

Otherwise sparse will complain about constants
that don't fit in "long" etc.

arch/x86_64/Makefile

@@ -37,7 +37,7 @@ LDFLAGS		:= -m elf_x86_64
 OBJCOPYFLAGS	:= -O binary -R .note -R .comment -S
 LDFLAGS_vmlinux := -e stext
 
-CHECKFLAGS      += -D__x86_64__=1
+CHECKFLAGS      += -D__x86_64__=1 -m64
 
 cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
 cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)