LoongArch: Include KBUILD_CPPFLAGS in CHECKFLAGS invocation

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.

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>