kbuild: enable 'make CPPFLAGS=...' to add additional options to CPP

The variable CPPFLAGS is a wellknown variable and the usage by
kbuild may result in unexpected behaviour.

This patch replace use of CPPFLAGS with KBUILD_CPPFLAGS all over the
tree and enabling one to use:
make CPPFLAGS=...
to specify additional CPP commandline options.

Patch was tested on following architectures:
alpha, arm, i386, x86_64, mips, sparc, sparc64, ia64, m68k, s390

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/ia64/Makefile b/arch/ia64/Makefile
index 9708a29..34951aa 100644
--- a/arch/ia64/Makefile
+++ b/arch/ia64/Makefile
@@ -29,7 +29,7 @@
 CFLAGS_KERNEL	:= -mconstant-gp
 
 GAS_STATUS	= $(shell $(srctree)/arch/ia64/scripts/check-gas "$(CC)" "$(OBJDUMP)")
-CPPFLAGS += $(shell $(srctree)/arch/ia64/scripts/toolchain-flags "$(CC)" "$(OBJDUMP)" "$(READELF)")
+KBUILD_CPPFLAGS += $(shell $(srctree)/arch/ia64/scripts/toolchain-flags "$(CC)" "$(OBJDUMP)" "$(READELF)")
 
 ifeq ($(GAS_STATUS),buggy)
 $(error Sorry, you need a newer version of the assember, one that is built from	\