arch: tile: include: asm: add cmpxchg64() definition

Need add cmpxchg64(), or will cause compiling issue.

Need define it as cmpxchg() only for 64-bit operation, since cmpxchg()
can support 8 bytes.

The related error (with allmodconfig):

  drivers/block/blockconsole.c: In function ‘bcon_advance_console_bytes’:
  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: Chris Metcalf <cmetcalf@tilera.com>