arch-arm: Fix WalkerState,Descriptors default constructor

arch-arm: Fix WalkerState,Descriptors default constructor

Those POD strutures are not initializing all members at construction.
This could lead to undefined behaviour

Change-Id: Iaa8afb126382b6bfbef686883a026262f24d5ca1
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Javier Setoain <javier.setoain@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19149
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>