These tests previously used "build/NULL" but due to changes in the
"Ruby" and "garnet_synth_traffic.py" scripts, "NULL" fails as the script
exists "X86TimingSimple" with MESI_Two_Level.
This change fixes the tests by compiling and using the correct
compilation of gem5. It shouldn't affect the tests in any negative way.
As far as I'm aware it does not matter what ISA is used for these tests.
Change-Id: I8ae84b49f65968e97bef4904268de5a455f06f5c