arm64: vdso32: Don't use KBUILD_CPPFLAGS unconditionally

KBUILD_CPPFLAGS is defined differently depending on whether the main
compiler is clang or not. This means that it is not possible to build
the compat vDSO with GCC if the rest of the kernel is built with clang.

Define VDSO_CPPFLAGS directly to break this dependency and allow a clang
kernel to build a compat vDSO with GCC:

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
    CROSS_COMPILE_COMPAT=arm-linux-gnueabihf- CC=clang \
    COMPATCC=arm-linux-gnueabihf-gcc

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>