arch-arm: Correct Illegal Exception Return detection

Fixed Illegal Exception Return detection, which was not
covering all the documented cases.

Change-Id: If08ddc1490d1c0a1fccee1489d116384770ce0a5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7223
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>