kbuild: Do not pick up ARCH_{CPP,A,C}FLAGS from the environment

Initialize the ARCH_* overrides before including the arch Makefile, to
avoid picking up the values from the environment. The variables can
still be overriden on the make command line, but this won't happen
by accident.

Signed-off-by: Michal Marek <mmarek@suse.com>

diff --cc Makefile
Simple merge