sim: add support for checkpoint downgrading
This commit supports the use case of transitioning tags and their

associated checkpoint rewrites out of use for whatever reason.  Just

replace the upgrader() method with a downgrader() method that performs

the appropriate inverse operation.
The tag name is still used, but only in this negative, 'zombie' state,

as it will be removed from the tags in the checkpoint and gem5 binary.
Change-Id: If9d26cccfe8449e026762b1a72f0c2ae5a9cf2d7

Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>