mips: Include KBUILD_CPPFLAGS in CHECKFLAGS invocation

A future change will move CLANG_FLAGS from KBUILD_{A,C}FLAGS to
KBUILD_CPPFLAGS so that '--target' is available while preprocessing.
When that occurs, the following error appears when building ARCH=mips
with clang (tip of tree error shown):

  clang: error: unsupported option '-mabi=' for target 'x86_64-pc-linux-gnu'

Add KBUILD_CPPFLAGS in the CHECKFLAGS invocation to keep everything
working after the move.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>