Also exclude 2mm, 3mm and gemm from benchmarking-only mode

These programs have really high variance in their execution time, and
frequently produce false positives. Exclude them from benchmarking
only mode until we figure out how to make them less noisy.

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@216860 91177308-0d34-0410-b5e6-96231b3b80d8