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>

ext/dramsim2/README

@@ -2,7 +2,7 @@ Follow these steps to get DRAMSim2 as part of gem5
 
 1. Download DRAMSim2
     1.1 Go to ext/dramsim2 (this directory)
-    1.2 Clone DRAMSim2: git clone git://github.com/dramninjasUMD/DRAMSim2.git
+    1.2 Clone DRAMSim2: git clone git@github.com:umd-memsys/DRAMSim2.git
 
 2. Compile gem5
     2.1 Business as usual

ext/dramsim2/SConscript

@@ -73,7 +73,7 @@ DRAMFile('Transaction.cpp')
 # DRAMSim2 violates some of the warning flags used by gem5, so
 # we explicitly disable them here
 dramenv = main.Clone()
-dramenv.Append(CCFLAGS=['-Wno-unused-value'])
+dramenv.Append(CCFLAGS=['-Wno-unused-value', '-Wno-error=nonnull-compare'])
 
 # If we are using clang, there are more flags to disable
 if main['CLANG']:

src/mem/DRAMSim2.py

@@ -34,7 +34,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 from m5.params import *
-from AbstractMemory import *
+from m5.objects.AbstractMemory import *
 
 # A wrapper for DRAMSim2 multi-channel memory controller
 class DRAMSim2(AbstractMemory):