sim: Move the serialization backend handlers to their own header.

sim: Move the serialization backend handlers to their own header.

This way other types which want to enable serialization can include just
these handlers and specialize them as necessary without bringing in all
the other dependencies of the serialization mechanism.

Change-Id: I7310e7741615e23ac0fc762e951bf5eac00aaa74
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36281
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>