arch: sparc: fix memory barrier behavior of arch_irq_*lock

Correct functioning of spinlocks requires that they be memory barriers.
Most architectures achieve this by using the GCC extended asm syntax to
force a compiler soft barrier at the point the interrupt status is
changing.  This clobber was missing from the SPARC definition, so add
it.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>