sim: Optionally pass "position" to GuestABI::Result::store.

sim: Optionally pass "position" to GuestABI::Result::store.

This will let it get at information about the signature as a whole.

Also, put result storing and argument getting behind functions to hide
some of the templating involved in those mechanisms.

Change-Id: Ib9f26ff69495f8891435f68d3d2f9dfa761a0274
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24105
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>