fastmodel: Move ARM but not CortexA76 specific bits to the IRIS TC.

fastmodel: Move ARM but not CortexA76 specific bits to the IRIS TC.

Now that the IRIS thread context can be specific to ARM, some things
which had been pushed to a different level of abstraction can be mvoed
back. This will hopefully allow more code sharing in the future when
other types of CPUs are supported.

Change-Id: Ic3a5f0db53ebe93e18f7507ed71812bce27b6d01
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23788
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>