cpu: Move the instruction port into o3's fetch stage.

cpu: Move the instruction port into o3's fetch stage.

That's where it's used, and that avoids having to pass it around using
the top level getInstPort accessor.

Change-Id: I489a3f3239b3116292f3dcd78a3945fb468c6311
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20239
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>