tests: Run qom-test for every architecture

Rather than requiring every new architecture to remember to add a line
to the Makefile to say that qom-test will work on it, autogenerate
the list of supported architectures by looking at the files in
default-configs (as configure does), and add qom-test to the
test list for all of them automatically.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>