ruby: reset and dump stats along with reset of the system

ruby: reset and dump stats along with reset of the system
This patch adds support to ruby so that the statistics maintained by ruby
are reset/dumped when the statistics for the rest of the system are
reset/dumped. For resetting the statistics, ruby now provides the
resetStats() function that a sim object can provide. As a consequence, the
clearStats() function has been removed from RubySystem. For dumping stats,
Ruby now adds a callback event to the dumpStatsQueue. The exit callback that
ruby used to add earlier is being removed.

Created by: Hamid Reza Khaleghzadeh.
Improved by: Lluc Alvarez, Nilay Vaish
Committed by: Nilay Vaish