sim: Split up the guest_abi.hh header.

sim: Split up the guest_abi.hh header.

This header was getting pretty long, and could be broken up into a few
headers which logically grouped related definitions and concepts.

To maintain compatibility and keep things simple for users of the
mechanism, there is still a top level header with the original name
which defines the interface for using ABIs. It includes all the other
new headers, and so can also be used when defining ABIs.

Change-Id: I62a051b9bd982e0fcecfceeb3d658d1ff4d30c5e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24104
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>