misc: Add support for switching multiple cores in SystemC

misc: Add support for switching multiple cores in SystemC

This patch adds a '-n' flag to the gem5 SystemC driver which allows
multiple CPUs to be switched out to a new CPU. Primarily this involves
appending CPU numbers to the objects searched for in the config
manager if there are multiple CPUs in the system.

Note that an equivalent change should be made to the util/cxx_config driver,
but I wanted to get input on this first before making the same change over
there

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>