[SPARC64]: Make cpu_present_map available earlier.

The change to kernel/sched.c's init code to use for_each_cpu()
requires that the cpu_possible_map be setup much earlier.

Set it up via setup_arch(), constrained to NR_CPUS, and later
constrain it to max_cpus in smp_prepare_cpus().

This fixes SMP booting on sparc64.

Signed-off-by: David S. Miller <davem@davemloft.net>