stdlib: Add checkpoint to Simulator

This change modifies the Simulator constructor to optionally
take a checkpoint directory as an input so that the m5 can
instantiate from the saved checkpoint.

A new method is also added to the Simulator class. The function
will save the checkpoint to the specified directory.

Change-Id: I58b686b6b4f69260ab45709c6ef0bddf4539f0c4
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58789
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed