asm-generic: move cmpxchg*_local defs to cmpxchg.h

asm/cmpxchg.h can be included on its own and needs to be self-consistent.
The definitions for the cmpxchg*_local macros, as such, need to be part
of this file.

This fixes a build issue on OpenRISC since the system.h smashing patch
96f951edb1f1bdbbc99b0cd458f9808bb83d58ae that introdued the direct inclusion
asm/cmpxchg.h into linux/llist.h.

CC: David Howells <dhowells@redhat.com>
Signed-off-by: Jonas Bonn <jonas@southpole.se>
Acked-by: Arnd Bergmann <arnd@arndb.de>