arch-arm: Simplify AMO code generation templates

arch-arm: Simplify AMO code generation templates

This change simplifies the isa template for the atomic memory
operation (AMO). Previously the flow had unecessary if statements that
ended up breaking build using clang, due to variables that could
seemingly be used before they were unitialized.

Change-Id: I1b46dfd5f1e90377245c4f649c08b6532b507b9c
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22603
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>