zdtm: autodetect the target machine v2

Andrew Vagin pointed out that the static testsuite compilation
may run as standalone so the variable ARCH won't be available.

This patch introduces the facility test/zdtm/Makefile.inc
designated to detect the target machine automatically
if the variable ARCH isn't defined. This makefile is supposed
to be included into all test subsuite Makefiles.

Andrew Vagin also pointed out that machine-dependent tests
may be added into non-static subsuites as well so the macro
adding machine-specific headers into CPPFLAGS goes to
the file test/zdtm/Makefile.inc.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>