test/zdtm/Makefiles: disable built-in rules

MAKEFLAGS += -r only works for sub-make, and it is not applicable to
the current instance. Since previous commit make is not re-running
itself (after re-reading deps files), so MAKEFLAGS no longer works.
Use one more way to disable built-in rules that stand in our way.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/Makefile.inc

+.SUFFIXES:
 MAKEFLAGS	+= -r
 
 ARCH ?= $(shell uname -m | sed          \