locking,arch,metag: Fold atomic_ops

Many of the atomic op implementations are the same except for one
instruction; fold the lot into a few CPP macros and reduce LoC.

This also prepares for easy addition of new ops.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: James Hogan <james.hogan@imgtec.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-metag@vger.kernel.org
Link: http://lkml.kernel.org/r/20140508135852.453864110@infradead.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>