sh: Overhaul spinlocks and rwlocks for SH-4A SMP.

This rips out some of the old spinlock and rwlock behaviour that the SH-2
parts were using and reworks them for LL/SC semantics on the SH-4A.

This is primarily only useful for SH-X3 multi-cores, but can also be used
for building CONFIG_SMP=y testing kernels on SH-4A UP.

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