sparc64: Define VA hole at run time, rather than at compile time.

Now that we use 4-level page tables, we can provide up to 53-bits of
virtual address space to the user.

Adjust the VA hole based upon the capabilities of the cpu type probed.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Bob Picco <bob.picco@oracle.com>