m68k: Add missing cmpxchg64() if CONFIG_RMW_INSNS=y

If CONFIG_RMW_INSNS=y:

drivers/block/blockconsole.c: In function ‘bcon_advance_console_bytes’:
drivers/block/blockconsole.c:164: error: implicit declaration of function ‘cmpxchg64’

Map cmpxchg64 to cmpxchg64_local, which is already mapped to
__cmpxchg64_local_generic, just like for the CONFIG_RMW_INSNS=n case.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>