LoongArch: Include KBUILD_CPPFLAGS in CHECKFLAGS invocation

mainline inclusion
from mainline-v6.5-rc1
commit 5ddc7a37
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7MWTU


CVE: NA

--------------------------------

This is a port of commit 08f6554f ("mips: Include KBUILD_CPPFLAGS in
CHECKFLAGS invocation") to arch/loongarch, for fixing cross-compilation
of Linux/LoongArch with Clang, where previously the `--target` flag
would no longer be present for the CHECKFLAGS cc invocation leading to
build failure.

Signed-off-by: Yongzhen Zhang <zhangyongzhen@kylinos.cn>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/1787#issuecomment-1608306002


Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>