m68k: Wire up membarrier

$ ./membarrier_test
membarrier MEMBARRIER_CMD_QUERY syscall available.
membarrier: MEMBARRIER_CMD_SHARED success.
membarrier: tests done!
$

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@uclinux.org>