sh: Fix up futex implementation.

SH is able to support a complete futex implementation on UP by way
of gUSA. However, IRQ toggling must be done for the old CPUs that
don't have movli.l/movco.l (LL/SC) instructions. Provide a default
implementation that does this, so it's possible to optimize for
newer CPUs.

Follows the same scheme as the current asm-sh/atomic-*.h headers.

Signed-off-by: Kaz Kojima <kkojima@rr.iij4u.or.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>