xen: arm: provide and use a handle_raz_wi helper

xen: arm: provide and use a handle_raz_wi helper

Reduces the use of goto in the trap handlers to none.

Some explicitly 32-bit types become register_t here, but that's OK, on
32-bit they are 32-bit already and on 64-bit it is fine/harmless to
set the larger register, a 32-bit guest won't see the top half in any
case.

Per section B1.2.1 (ARMv8 DDI0487 A.d) writes to wN registers are zero
extended, so there is no risk of leaking the top half here.

Unlike the previous code the advancing of PC is handled within the
helper, rather than after the end of the switch as before. So return
as the handler is called.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>