cpu: Stop using the ThreadContext::nextInstAddr method.

The PCState already contains this information internally, and it can be
compared, printed, etc, implicitly alongside all the other info in the
PCState, everywhere this method was being used.

Change-Id: I30705f30a135d4ffbc3279c366dafb1184445c70
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52052
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed