kbuild: hdrcheck: fix cross build with clang

The headercheck tries to call clang with a mix of compiler arguments
that don't include the target architecture. When building e.g. x86
headers on arm64, this produces a warning like

   clang: warning: unknown platform, assuming -mfloat-abi=soft

Add in the KBUILD_CPPFLAGS, which contain the target, in order to make it
build properly.

See also 1b71c2fb ("kbuild: userprogs: fix bitsize and target
detection on clang").

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Fixes: feb843a4 ("kbuild: add $(CLANG_FLAGS) to KBUILD_CPPFLAGS")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>