blackfin: fix cmpxchg build fails from system.h fallout

Commit 3bed8d67 ("Disintegrate asm/system.h for Blackfin [ver #2]")
introduced arch/blackfin/include/asm/cmpxchg.h but has it also including
the asm-generic one which causes this:

  CC      arch/blackfin/kernel/asm-offsets.s
  In file included from arch/blackfin/include/asm/cmpxchg.h:125:0,
                 from arch/blackfin/include/asm/atomic.h:10,
                 from include/linux/atomic.h:4,
                 from include/linux/spinlock.h:384,
                 from include/linux/seqlock.h:29,
                 from include/linux/time.h:8,
                 from include/linux/timex.h:56,
                 from include/linux/sched.h:57,
                 from arch/blackfin/kernel/asm-offsets.c:10:
  include/asm-generic/cmpxchg.h:24:15: error: redefinition of '__xchg'
  arch/blackfin/include/asm/cmpxchg.h:82:29: note: previous definition of '__xchg' was here
  make[2]: *** [arch/blackfin/kernel/asm-offsets.s] Error 1

It really only needs two simple defines from asm-generic, so just use
those instead.

Cc: Bob Liu <lliubbo@gmail.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/blackfin/include/asm/cmpxchg.h

@@ -122,7 +122,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 			(unsigned long)(n), sizeof(*(ptr))))
 #define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
 
-#include <asm-generic/cmpxchg.h>
+#define cmpxchg(ptr, o, n)	cmpxchg_local((ptr), (o), (n))
+#define cmpxchg64(ptr, o, n)	cmpxchg64_local((ptr), (o), (n))
 
 #endif /* !CONFIG_SMP */