linux-user: Don't return uninitialized value for atomic_barrier syscall

QEMU's implementation of the m68k atomic_barrier syscall, like the kernel's,
is just a no-op. However we still need to return a result code from it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
