sh: Provide a non-multiplexed sys_recvmmsg path.

Now that the rest of the socket calls are provided through their own
paths, do the same for sys_recvmmsg. It's unlikely we'll ever be able to
kill off the socketcall path, but this at least permits userspace to
gradually begin migrating.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>