sim: Move findFreeContext to System.

sim: Move findFreeContext to System.

This method searches through the ThreadContexts stored in the system,
and has no concrete connection to Process other than it happened to be
used by a Process in the clone system call.

By moving it, we can use its functionality in classes other than
Process.

Change-Id: Ic6899c335dc13841c6fe948ea3a4f8ad67e562bb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24285
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>