tests: Add Makefiles for hello

This adds Makefiles for hello for ARM and x86 by leveraging docker and
dockcross. See https://github.com/dockcross/dockcross for more
information.

These Makefiles also allow for automatic uploading to the correct location
for users to download when running the new tests.

Change-Id: I7085000393cd5283502a7af362c85befda749181
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/4883