arch-arm: Be more verbose on load/store construction

arch-arm: Be more verbose on load/store construction

This is achieved by using keyword arguments to improve readibility.
Some of the building helpers are using native types and can be annoying
for a reader to understand what those sequences of number and boolean
mean. It is also easier in this way to commit mistakes.

Change-Id: I63081d09a1f621550c5b6522b8107f349939b21d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24044
Tested-by: kokoro <noreply+kokoro@google.com>