Follow these steps to get DRAMSim3 as part of gem5

1. Download DRAMSim3
    1.1 Go to ext/dramsim3 (this directory)
    1.2 Clone DRAMSim3: git clone git@github.com:umd-memsys/DRAMSim3.git DRAMsim3
    1.3 cd DRAMSim3 && mkdir build
    1.4 cd build
    1.5 cmake ..
    1.6 make

2. Compile gem5
    2.1 cd gem5
    2.2 Business as usual