stdlib: Refactor Looppoint

This change refactors the Looppoint files. While functionally
equivalent, this classes have been moved and altered to be easier to
handle going forward. The following changes have been made:

- New classes have been added to represent the data structure of the
  Looppoint JSON. This simplifies the parsing of JSON files and makes it
  handle Looppoint data structures. Ultimately this is hidden from the
  user via the new 'gem5.resources.Looppoint' class which will be the
  front-facing class for Looppoint interactions.
- The `LooppointCheckpoint` class has been replaced with
  `LooppointCsvLoader`. This new class takes in a CSV pintpoints file
  to load necessary looppoint data.
- The `LoopPointRestore` class has been replaced by
  `LooppointJsonLoader`.
- All Looppoint classes have been moved to `gem5.resources`. This will
  make it easier when we add Looppoints as specific gem5 resources.

Change-Id: I11dd1fe8f76658db220320584270d57cb37a3c62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67611
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
7 files changed