cpu: Fixed unused var error when with fast builds

cpu: Fixed unused var error when with fast builds

As `is_htm_speculative` is only used in assert statements, it is
considered unused during the `.fast` compilation. This commit adds the
`M5_USED_VAR` macro.

This caused our compiler tests to fail:
https://www.mail-archive.com/gem5-dev@gem5.org/msg35913.html

Change-Id: I00d187d1a31d065c236ac29a657bd479ad4b03bc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34256
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>