kbuild: Set KBUILD_CFLAGS before incl. arch Makefile

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 <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Reviewed-by: Matthias Kaehlcke <[email protected]>
Tested-by: Matthias Kaehlcke <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>