fastmodel: Ignore clearArchRegs for now.

fastmodel: Ignore clearArchRegs for now.

This only seems to be used from outside of the CPU when resetting state
at the start of execution. Since this state is already reset in
fast model, we can mostly ignore that call for now.

When more accessors are implemented, this function can be use them to
clear registers like it would on other thread contexts.

Change-Id: I5146273387ec17987770abc67f6f426c4480e0b9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24967
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Maintainer: Gabe Black <gabeblack@google.com>