x86/purgatory: use approprate -m64/-32 build flag for arch/x86/purgatory

Thomas reported that build of x86_64 kernel was failing for him.  He is
using 32bit tool chain.

Problem is that while compiling purgatory, I have not specified -m64
flag.  And 32bit tool chain must be assuming -m32 by default.

Following is error message.

(mini) [~/work/linux-2.6] make
scripts/kconfig/conf --silentoldconfig Kconfig
  CHK     include/config/kernel.release
  UPD     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
  UPD     include/generated/utsrelease.h
  CC      arch/x86/purgatory/purgatory.o
arch/x86/purgatory/purgatory.c:1:0: error: code model 'large' not supported in
the 32 bit mode

Fix it by explicitly passing appropriate -m64/-m32 build flag for
purgatory.
Reported-by: Thomas Glanzmann <thomas@glanzmann.de>
Tested-by: Thomas Glanzmann <thomas@glanzmann.de>
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/purgatory/Makefile

@@ -11,6 +11,7 @@ targets += purgatory.ro
 # sure how to relocate those. Like kexec-tools, use custom flags.
 
 KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes -fno-zero-initialized-in-bss -fno-builtin -ffreestanding -c -MD -Os -mcmodel=large
+KBUILD_CFLAGS += -m$(BITS)
 
 $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
 		$(call if_changed,ld)