arch,cpu: Convert ExecContext::pcState to use PCStateBase.

Some places need persistent temporaries for the return values of
ThreadContext::pcState(), which is currently by value.

Change-Id: Icd4924f1d16ebe1c99c54ed47616733422340cfe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52057
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
12 files changed