tests/migration: Enable the migration test on s390x, too

tests/migration: Enable the migration test on s390x, too

We can re-use the s390-ccw bios code to implement a small firmware
for a s390x guest which prints out the "A" and "B" characters and
modifies the memory, as required for the migration test.

[quintela: Converted the compile script to Makefile rules]
Signed-off-by: Juan Quintela <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1539078677[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
  Fixed up Makefile since the aarch patch sneaked in first