arm: Use the "reg" ABI for gem5 ops.

arm: Use the "reg" ABI for gem5 ops.

The generic PseudoInstABI just calls back into the ISA specific
getArgument function, and that adds a lot of handling for cases that
aren't used and, besides those, basically just boils down to the "reg"
ABI anyway.

Change-Id: I57e738631dbccbf89cba3a6ca62b1f954b39e959
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39316
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>