kbuild: Set KBUILD_CFLAGS before incl. arch Makefile

Set the clang KBUILD_CFLAGS up before including arch/ Makefiles,
so that ld-options (etc.) can work correctly.

This fixes errors with clang such as ld-options trying to CC
against your host architecture, but LD trying to link against
your target architecture.

Signed-off-by: Chris Fries <cfries@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Tested-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>