sim: Added M5_VAR_USED to unused `cpu` var

sim: Added M5_VAR_USED to unused `cpu` var

The `BaseCPU cpu` variable unused when compiling gem5.fast. This
causes the compilation to fail. Adding the M5_VAR_USED resolves this
issue.

Change-Id: I62588563e9cde384e30755742d6bc754e819d7f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31214
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>