cpu,o3: Fixed checkpointing bug occuring in the o3 CPU

cpu,o3: Fixed checkpointing bug occuring in the o3 CPU

Checkpointing a system with out-of-order CPUs might get stuck if
one of the CPUs has been put to sleep. The quiesce instruction
cannot get drained hence checkpointing never finishes.

This commit resolves that by activating all suspended thread
contexts when draining the system.

Change-Id: I817ab1672b4ead777bd8e12a0445829481c46fdc
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3970
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>