arch-arm: Rewrite the currEL helper method to use opModeToEL

arch-arm: Rewrite the currEL helper method to use opModeToEL

Direct use of cpsr.el should be discouraged: it should be used when
in AArch64 only; when in AArch32 it won't return the matching EL.

Eg: when in Supervisor Mode (EL1), CPSR.M<3,0> (mode) is 0b0011,
and cpsr.el will return 0 (EL0)

Change-Id: I5504bd1f59980f79b2607cce435ea09245de12e5
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/+/20249
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>