arch: cpu: Make the ExtMachInst type a template argument in InstMap.

arch: cpu: Make the ExtMachInst type a template argument in InstMap.

This doesn't completely hide the ISA specific ExtMachInst type inside
the ISAs since it still gets applied in arch/generic, but it at least
pulls it into the arch directory.

Change-Id: Ic2188d59696530d7ecafdff0785d71867182701d
Reviewed-on: https://gem5-review.googlesource.com/9403
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>