arch-arm: Don't change PSTATE in Illegal Exception return

arch-arm: Don't change PSTATE in Illegal Exception return

This patch fixes the Illegal Exception return handler. According to the
armarm documentation, when PSTATE.IL is set to one because of an illegal
exception return, PSTATE.{EL, nRW, SP} are unchanged. This means the
Exception level, Execution state, and stack pointer selection do not
change as a result of the return.

Change-Id: I35f2fe68fb2822a54fc4a21930871eab7a1aaab4
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8021
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>