FRV: Don't make smp_{r, w, }mb() interpolate MEMBAR when CONFIG_SMP=n [try #2]

Don't make smp_{r,w,}mb() interpolate a MEMBAR instruction when CONFIG_SMP=n as
SMP memory barries on UP systems should interpolate a compiler barrier only.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>