sh64: wire up sys_accept4.

sh64 on the other hand provides both direct broken out syscalls as well
as socketcall access. As there are binaries that use both socketcall has
to stay around. The current ABI prefers direct syscalls.

It was pointed out that when sys_recvmmsg was added in, sys_accept4 was
overlooked. This takes care of wiring it up.

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