[v2] linux-user: implement m68k atomic syscalls

With nptl enabled, atomic_cmpxchg_32 and atomic_barrier
system calls are needed. This patch enabled really dummy
versions of the system calls, modeled after the m68k
kernel code.

With this patch I am able to execute m68k binaries
with qemu linux-user (busybox compiled for coldfire).

[v2] que an segfault instead of returning a EFAULT
to keep in line with kernel code.

Cc: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>