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: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>