sh: Avoid exporting unimplemented syscalls.

Now that userspace is making use of kernel-provided sanitized headers for
working out supported interfaces, we need to be a bit more diligent with
matching the syscall definitions with their actual wiring/support state.

In theory it shouldn't hurt anything since sys_ni_syscall will ultimately
do the right thing, but there's also not much need to lie about legacy
x86 syscalls that we've never supported.

This tightens things up a bit for uClibc at least.

Suggested-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>