powerpc: Optimise smp_wmb

Change 2d1b2027 ("powerpc: Fixup
lwsync at runtime") removed __SUBARCH_HAS_LWSYNC, causing smp_wmb to
revert back to eieio for all CPUs.  This restores the behaviour
intorduced in 74f06095

 ("powerpc:
Optimise smp_wmb on 64-bit processors").

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>