Fix invalid profile for command-completer in remote-sim.c

Fix invalid profile for command-completer in remote-sim.c

The profile of command completers has been change to returna VEC
of char_ptr.  Most completers were updated, except the one in
remote-sim.c.

Unfortunately, to make things a little more difficult, the meat of
the completer is actually implemented in the sim, were VECs are not
available.  This patch thus translates the returned array into a VEC,
and then returns that VEC.

gdb/ChangeLog:

        * remote-sim.c (sim_command_completer): Change type of return
        value to "VEC (char_ptr) *".  Adjust implementation accordingly.