sim,cpu: make exit_group halt all threads in a group

sim,cpu: make exit_group halt all threads in a group

When a thread calls exit_group, in addition to halting the thread
itself, it needs to halt all other threads in its group (i.e., threads
sharing the same thread group ID). This patch enables threads to do
that.

Change-Id: Ib2e158fb27cf98843f177a64a2d643b1bbc94d03
Reviewed-on: https://gem5-review.googlesource.com/c/9623
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>