sim-se: Add /sys/devices/system/cpu/online file

sim-se: Add /sys/devices/system/cpu/online file

Add the special file /sys/devices/system/cpu/online to the files that gem5
knows how to handle in SE mode. This file lists the CPUs that are active.
For instance, in an 8 CPU system it is the following:
0-7

This implementation simply returns a file that is 0-%d where %d is the
current number of thread contexts.

This file is required for C++11 threads with gcc 4.8 and above.

Change-Id: I0b566f77e75e9eca480509814d0fd038a231b940
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/8902
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>