Add cmpxchg_local to sparc, move __cmpxchg to system.h

Move cmpxchg and add cmpxchg_local to system.h.
Use the new generic cmpxchg_local (disables interrupt).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>