cpu: Avoid unnecessary dynamic_pointer_cast in atomic model

cpu: Avoid unnecessary dynamic_pointer_cast in atomic model

In the atomic model a dynamic_pointer_cast is performed at every tick to
check if the fault is a SyscallRetryFault. This was happening even when
there was no generated fault.

Change-Id: I7f4afeffffdf4f988230e05286602d8d9a919c6c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10101
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>