stdlib: Rename JsonSerializable to SerializableStat

As this abstract class now allows the output of text stats, it's more
appropriate to rename it. It no longer handles processing just for JSON
output

Change-Id: Ia9a1e3ef4029de45a11ac261fb14c9bdfa412cdd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59273
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
7 files changed