configs,sim-se: fix se.py multi-cpu multi-cmd issue

configs,sim-se: fix se.py multi-cpu multi-cmd issue

Assign different pids to the different commands specified with the "--cmd"
flag to configs/example/se.py

Without this change, the following command line triggers
a "fatal: _pid 100 is already used" error:

command=$PWD/tests/test-progs/hello/bin/arm/linux/hello
./build/ARM/gem5.opt configs/example/se.py -n 2 -c "$command;$command"

Change-Id: If6f726481eb196d4f42680b6aa46364fce4190ed
Signed-off-by: Pau Cabre <pau.cabre@metempsy.com>
Reviewed-on: https://gem5-review.googlesource.com/4160
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>