arch-arm: Replace direct use cpsr.el with currEL helper

arch-arm: Replace direct use cpsr.el with currEL helper

The patch is replacing it in places where the current EL could be using
AArch32, hence leading to an incorrect ExceptionLevel.

Change-Id: I99b75af2668f2c38fd88bec62e985ab7dbea80dc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20251
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>