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: David S. Miller <davem@davemloft.net>