cpu: Use cprintf and C++ type magic to get rid of a THE_ISA.

cpu: Use cprintf and C++ type magic to get rid of a THE_ISA.

It should be fine to let operator overloading take care of figuring out
how to print the ExtMachInst type for a given ISA.

Change-Id: I173fd9f49013d92191118775d20344219a69337e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34822
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>