barrier.h: add generic smp_mb implementation
barrier.h: add generic smp_mb implementation

This missing define causes build failures on s390:

  src/include/liburing.h:298: undefined reference to `io_uring_smp_mb'

Signed-off-by: Jeff Moyer <cd6badf7869d996b866599075536a600ed1910b5@redhat.com>
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing