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 <mathieu.desnoyers@polymtl.ca>
Cc: Bryan Wu <bryan.wu@analog.com>
Cc: Michael Frysinger <michael.frysinger@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>