fastmodel: Move the ARM IRIS threadcontext into CortexA76.

fastmodel: Move the ARM IRIS threadcontext into CortexA76.

This specialization will correspond specifically with the CortexA76,
instead of specializing the ThreadContext for ARM in general. Some
aspects of this class may need to move into the base IRIS thread
context class, but I'll leave that for a later change.

Change-Id: I9cbe527d36e6fda78601dc39c1963370cfa28b16
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23787
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>