fastmodel: Set itstate when building a PCState from IRIS.

fastmodel: Set itstate when building a PCState from IRIS.

These bits are probably never going to be non-zero since we'd have to
take a checkpoint part way through an if/then construct in thumb, but
they're easy to extract and we might as well store them properly.

Change-Id: Ifc5c34063dd23f72cc106c0d77d90c5e6ee871be
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24328
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>