sim: add sim_complete_command stubs for non-common-using ports

sim: add sim_complete_command stubs for non-common-using ports

For the ports that don't use the common/ subdir, we need to add stub funcs
to them to avoid build failures with gdb and command completion.  These do
not implement the actual completion functionality ... any port that wants
that can either convert to the common/ subdir, or fill out the function on
their own time.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>