Support non-block prefetch inst in Ruby

[seanzw]

• Problem
  By default, the prefetch instruction in x86 is implemented as normal load instruction (with prefetch flag in the request), and blocks the O3 cpu to wait for the response when we are using Ruby. This causes performance problem as the latency is exposed to the pipeline.

• Possible Solution
  Currently, the classic cache returns early for the prefetch request. We could also implement this behavior in the RubySequencer. This would fix this issue for all CPU models and Ruby protocols.