atomic.h: add atomic64 cmpxchg, xchg and add_unless to alpha

This series mainly adds support for missing 64 bits cmpxchg and 64 bits atomic
add unless.  Therefore, principally 64 bits architectures are targeted by
these patches.  It also adds the complete list of atomic operations on the
atomic_long type.

This patch:

atomic.h: add atomic64 cmpxchg, xchg and add_unless to alpha

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>