sim: mloop: mark a few conditionally used funcs as unused

sim: mloop: mark a few conditionally used funcs as unused

These are marked inline, so building w/gcc at higher optimization
levels will automatically discard them.  But building with -O0 will
trigger unused function warnings, so fix that.

The common before/after cover functions in the common mloop generator
are not used by all architecture ports.  Doesn't seem to be a hard
requirement, so marking them optional (i.e. unused) is fine.

The cris execute function is conditionally used depending on the
fast-build mode settings, so mark it unused too.