sim: Make it possible for a GuestABI to init its Position based on a TC.

sim: Make it possible for a GuestABI to init its Position based on a TC.

It may be necessary to initialize the GuestABI Position type based on
the current state of the thread, for instance by reading the current
stack pointer.

This change makes it possible (but not mandantory) for an ABI to supply
a constructor for Position which accepts a ThreadContext * which it can
use to intiialize itself.

Change-Id: I5609b185f746368c5f9eb2a04074dcafa088f925
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23749
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>