@cpu@ in generated sim files

[Ben Elliston]

I'd like to propose that we move in the general direction of renaming @cpu@
to @prefix@ in the generated sim files.  The prefix can then be substituted
in by the user (or Makefiles on their behalf) in a more predictable fashion.
This would just make things more consistent throughout.  Of course, in some
cases, there is a need for @arch@, @cpu@, et al. and these cases would need
to be considered.

Thoughts?

Ben