kbuild: Stop using '-Qunused-arguments' with clang

This option masks all unused command line argument warnings, which can
hide potential issues, such as an architecture Makefile adding an
unsupported flag to KBUILD_AFLAGS or KBUILD_CFLAGS, which will cause all
as-option and cc-options to silently fail due to -Werror with no
indication as to why in the main kernel build.

Remove this flag so that warnings of this nature can be caught early and
obviously in a build.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>