cpu: Fix usage of setArchVecElem

cpu: Fix usage of setArchVecElem

setArchVecElem should create a VecElemClass RegId, and not a VecRegClass.
Initializing a VecRegClass with three arguments makes it panic

Change-Id: I6c398d67305bfe7bea12cb02edd4f4c3a202e69a
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15655
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>