sparc32: enable different preemptions models

While chasing following warning from kconfig I noticed that the
kconfig preemption model symbols were all dependent on sparc64.

warning: (PREEMPT && DEBUG_ATOMIC_SLEEP) selects PREEMPT_COUNT which has unmet direct dependencies (SPARC64)

>From arch/sparc/Kconfig:

        if SPARC64
        source "kernel/Kconfig.preempt"
        endif

But looking a bit closer I see nothing obvious why
sparc32 should not support the various preemption models.
Drop the "if SPARC64" conditional to enable selection of
preemption model on sparc32 too.

Build-tested - but not run-time tested all three models.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>