Add cmpxchg_local to blackfin, replace __cmpxchg by generic cmpxchg

Add cmpxchg_local to blackfin, replace __cmpxchg by generic cmpxchg

Use the new generic cmpxchg_local (disables interrupt). Also use the generic
cmpxchg as fallback if SMP is not set since nobody seems to know why __cmpxchg
has been implemented in assembly in the first place thather than in plain C.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Bryan Wu <[email protected]>
Cc: Michael Frysinger <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>