zdtm: add compatible tests building

`make test COMPAT_TEST=y` will build all executable tests as i386 ELFs.

For building tests, need the following libraries from multilib:
libaio.i686 libaio-devel.i686 libcap-devel.i686 libcap.i686

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/Makefile

 SUBDIRS	=	lib static transition
 
+ifeq ($(COMPAT_TEST),y)
+ifeq ($(ARCH),x86)
+        export USERCFLAGS += -m32
+        export CFLAGS += -m32
+        export LDFLAGS += -m32
+endif
+endif
+
 default:	all
 .PHONY: default lib static transition