Clang can assemble these files just fine; this is a relic from the top
level Makefile conditionally adding this. We no longer need --prefix,
--gcc-toolchain, or -Qunused-arguments flags either with this change, so
remove those too.

To test building:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
  CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 LLVM_IAS=1 \
  defconfig arch/arm64/kernel/vdso32/
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210420174427.230228-1-ndesaulniers@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>