[Gc] libatomic-ops nios2

Hello,

I am trying to add support for nios2 CPU into libatomic-ops and gc .
The Linux kernel exports kuser helper for cmpxchg [1], so I believe
this might be the way to go? Or shall I somehow use the gcc atomic
builtin ops [2] ? Note that nios2 does not have dedicated atomic
instructions which could be used in userland, that's why the kuser
helper.

Thank you for any input!

[1] http://lxr.free-electrons.com/source/arch/nios2/kernel/entry.S#L535
[2] https://github.com/gcc-mirror/gcc/blob/master/libgcc/config/nios2/linux-
atomic.c

Best regards,
Marek Vasut