kbuild: Fix 4.9.138 mismerge

740f140b was intended to move the clang KBUILD_CFLAGS
before arch Makefile include, but because of merge issue it
created an another set of CFLAGS that overlapped the old one.
Let's do it the way it should be done.

Change-Id: Iddc2aae1f870023e74feb28ccf56e439dd1facf6
Signed-off-by: Artem Borisov <dedsa2002@gmail.com>