[SystemZ] Fix invalid assumption in getCPUNameFromS390Model

Code in getCPUNameFromS390Model currently assumes that the
numerical value of the model number always increases with
future hardware.  While this has happened to be the case
with the last few machines, it is not guaranteed -- that
assumption was violated with (much) older machines, and
it can be violated again with future machines.

Fix by explicitly listing model numbers for all supported
machine models.