benchmark: pass execArgv to the benchmarking process

Benchmarker should pass exec flags (e.g. --no-crankshaft,
--turbofan-filter, --trace-opt etc) to the benchmarking process

Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
PR-URL: https://github.com/iojs/io.js/pull/928