arch/tile: Fix bug in support for atomic64_xx() ops.

This wasn't properly tested until the perf-event subsystem started
to get brought up under the tile architecture.

The bug caused bogus atomic64_cmpxchg() values to be returned,
among other things.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>