sh: convert to generic helpers for IPI function calls

This converts sh to use the new helpers for smp_call_function() and
friends, and adds support for smp_call_function_single(). Not tested,
but it compiles.

Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>