dev: Exit correctly in dist-gem5 for SE mode

dev: Exit correctly in dist-gem5 for SE mode

Do not allow the exit() syscall to terminate gem5 when running in dist-gem5
mode.  The exit must be coordinated by the distributed interface instead.

Change-Id: I57f47610b59fe9e18ba3a1667fb5e45cecac1a81
Reviewed-on: https://gem5-review.googlesource.com/10461
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Mohammad Alian <m.alian1369@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>