sparc64: Add __arch64__ to CHECKFLAGS

Otherwise sparse doesn't work.  The 32 vs. 64 header ifdef
used under arch/sparc/include/asm/ is:

#if defined(__sparc__) && defined(__arch64__)

And that doesn't work for sparse unless we give it __arch64__
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc64/Makefile

@@ -7,7 +7,7 @@
 # Copyright (C) 1998 Jakub Jelinek (jj@ultra.linux.cz)
 #
 
-CHECKFLAGS	+= -D__sparc__ -D__sparc_v9__ -m64
+CHECKFLAGS	+= -D__sparc__ -D__sparc_v9__ -D__arch64__ -m64
 
 # Undefine sparc when processing vmlinux.lds - it is used
 # And teach CPP we are doing 64 bit builds (for this case)