arch-arm: Fix incorrect assumptions in ELIs64

arch-arm: Fix incorrect assumptions in ELIs64

The state of EL1 wasn't determined correctly when running in secure
mode if virtualisation was enabled. This changset updates the
implementation to match the canonical behavior from the ARM ARM.

Change-Id: I7ed6f5c003617773603f678667aac069d73b6f62
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7141
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>