sim: create SERIALIZE_MAPPING and UNSERIALIZE_MAPPING

sim: create SERIALIZE_MAPPING and UNSERIALIZE_MAPPING

The motivation for those new methods is to prevent checkpoints from
breaking when new map entries are added.

Change-Id: I0ff8681498bcf669492e6b876ad385fda4673d77
JIRA: https://gem5.atlassian.net/browse/GEM5-661
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36135
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>