sim: Fixed error when compiling gem5 with dramsim2.

sim: Fixed error when compiling gem5 with dramsim2.

Compiling gem5 with dramsim2 included fails due to some inconsistencies in
including SimObjects. In this patch this issue is fixed along with
temporarily disabling -Werror=nonnull-compare in CCFLAGS. Also, the remote
for cloning dramsim2 has been changed.

Change-Id: Ia24095150d026d736352aaf0d735b7554ede10bb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31434
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>