Add options to help benchmark SDT

Add options to help benchmark SDT

These are secret, undocumented, for developer use only.  Use stap
inception-style as the -c CMD of a stap which probes the SDT mark
process.provider("stap").mark("benchmark").

  --benchmark-sdt-loops=N
        Loop over the SDT probe point N times.

  --benchmark-sdt-threads=N
        Run the loop simultaneously in N threads.