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>
diff --git a/arch/m68k/include/asm/cmpxchg.h b/arch/m68k/include/asm/cmpxchg.h
index 5c81d0e..bc755bc 100644
--- a/arch/m68k/include/asm/cmpxchg.h
+++ b/arch/m68k/include/asm/cmpxchg.h
@@ -124,6 +124,9 @@
 #define cmpxchg_local(ptr, o, n)					    \
 	((__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),	    \
 			(unsigned long)(n), sizeof(*(ptr))))
+
+#define cmpxchg64(ptr, o, n)	cmpxchg64_local((ptr), (o), (n))
+
 #else
 
 /*