arch: xtensa: include: asm: compiling issue, need cmpxchg64() defined.

When compiling with 'allmodconfig', some of drivers need cmpxchg64(),
xtensa does not supply 64-bit implementation for 'xchg', so use the
'generic' implementation.

e.g. (for next-20130527 tree):
  drivers/block/blockconsole.c:164:2: error: implicit declaration of function ‘cmpxchg64’ [-Werror=implicit-function-declaration]
Signed-off-by: NChen Gang <gang.chen@asianux.com>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/include/asm/cmpxchg.h

@@ -93,6 +93,7 @@ static inline unsigned long __cmpxchg_local(volatile void *ptr,
 	((__typeof__(*(ptr)))__cmpxchg_local_generic((ptr), (unsigned long)(o),\
 			(unsigned long)(n), sizeof(*(ptr))))
 #define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
+#define cmpxchg64(ptr, o, n)    cmpxchg64_local((ptr), (o), (n))
 
 /*
  * xchg_u32