powerpc: Optimise smp_wmb on 64-bit processors

For 64-bit processors, lwsync is the recommended method of store/store
ordering on caching enabled memory.  For those subarchs which have
lwsync, use it rather than eieio for smp_wmb.

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