O3: When waiting to handle an interrupt, let everything drain out.

Before this change, the commit stage would wait until the ROB and store queue
were empty before recognizing an interrupt. The fetch stage would stop
generating instructions at an appropriate point, so commit would then wait
until a valid time to interrupt the instruction stream. Instructions might be
in flight after fetch but not the in the ROB or store queue (in rename, for
instance), so this change makes commit wait until all in flight instructions
are finished.