arm: Use a non-template indexed version of laneView in aapcs32.

arm: Use a non-template indexed version of laneView in aapcs32.

The lane number is constant over its lifetime, but is computed with a
variable i which is not a compile time constant. It therefore can't be
used as a template parameter, and should be marked as const and not
constexpr.

Change-Id: Ie0b950311495831d5224a8fb397cf42d5cf5f25b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26834
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>