sim: Rename GuestABI's Position to State.

sim: Rename GuestABI's Position to State.

This type can hold any generic state related to locating return types
and arguments in addition to simple position information. To make that
clearer, this change renames the Position type to State.

Change-Id: I50ff2ec61c3eba0e9505c66ce32e27b515bd4b27
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24107
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>