[POWERPC] Implement PowerPC futex_atomic_cmpxchg_inatomic().

The sys_[gs]et_robust_list() syscalls were wired up on PowerPC but
didn't work correctly because futex_atomic_cmpxchg_inatomic() wasn't
implemented.  Implement it, based on __cmpxchg_u32().

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>