um: Build always with -mcmodel=large on 64bit

On UML SUBARCH can be x86, x86_64 and i386 and if it is x86
we use uname -m to select a defconfig.
Therefore we can no longer use -mcmodel=large only if SUBARCH
is x86_64.
Reported-and-tested-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Makefile

@@ -33,12 +33,11 @@ MODE_INCLUDE	+= -I$(srctree)/$(ARCH_DIR)/include/shared/skas
 
 HEADER_ARCH 	:= $(SUBARCH)
 
-# Additional ARCH settings for x86
-ifeq ($(SUBARCH),i386)
-        HEADER_ARCH := x86
+ifneq ($(filter $(SUBARCH),x86 x86_64 i386),)
+	HEADER_ARCH := x86
 endif
-ifeq ($(SUBARCH),x86_64)
-        HEADER_ARCH := x86
+
+ifdef CONFIG_64BIT
 	KBUILD_CFLAGS += -mcmodel=large
 endif