arch-arm: Change ArmFault cast from reinterpret to static

Changing casting type in src/arch/arm/isa.cc

Change-Id: Ia19b30a1bf8b1b25df149b52613a3533eaced03a
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8241
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>