s390/boot: fix boot of compressed kernel built with gcc 4.9

Add -fno-delete-null-pointer-checks to CFLAGS for the code in
arch/s390/boot. Without the option a compressed kernel built with
gcc 4.9 won't boot.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/compressed/Makefile

@@ -12,7 +12,7 @@ targets += misc.o piggy.o sizes.h head$(BITS).o
 
 KBUILD_CFLAGS := -m$(BITS) -D__KERNEL__ $(LINUX_INCLUDE) -O2
 KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
-KBUILD_CFLAGS += $(cflags-y)
+KBUILD_CFLAGS += $(cflags-y) -fno-delete-null-pointer-checks
 KBUILD_CFLAGS += $(call cc-option,-mpacked-stack)
 KBUILD_CFLAGS += $(call cc-option,-ffreestanding)