configs: Call createThreads() for FutureCPUs.

When using fast forwarding, createThreads() is not
called upon FutureCPUs. This causes segment fault
as the decoder is initialized in createThreads() and
needed when instantiating CPUs.

This commit basically fixes this by invoking
createThreads() on FutureCPUs after they are created.

Change-Id: I812d18f06878f9fc3fa2183a2c8a64d316413398
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56812
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Luming Wang <wlm199558@126.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/common/Simulation.py

@@ -488,6 +488,7 @@ def run(options, root, testsys, cpu_class):
                     options.indirect_bp_type)
                 switch_cpus[i].branchPred.indirectBranchPred = \
                     IndirectBPClass()
+            switch_cpus[i].createThreads()
 
         # If elastic tracing is enabled attach the elastic trace probe
         # to the switch CPUs