cpu: fix how branching is handled when a thread is suspended in MinorCPU

cpu: fix how branching is handled when a thread is suspended in MinorCPU

When a thread is suspended, all instructions after the suspension need
to be discarded since the thread will take a different execution stream
when it wakes up.

To do that, in MinorCPU, whenever a thread gets suspended, we change the
current execution stream by updating the current branch with
BranchData::SuspendThread reason.

Change-Id: I7cdcda22c1cf6e8ac8db8800b7d9ec052433fdf3
Reviewed-on: https://gem5-review.googlesource.com/c/9626
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@gmail.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>