EC: Allow bench.sh to test ISA backend also

Update bench.sh/plot tool to cover ISA backend.
ISA will output a fake echinique 'cauchy_good' so the plot tool
don't need to be changed.

Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>
Merge pull request #2868 from zhouyuan/bench_tool_isa

EC: Allow bench.sh to test ISA backend also

Reviewed-by: Loic Dachary <loic-201408@dachary.org>