sparc64: Add missing notify_cpu_starting() call.

Commit e545a6140b698b2494daf0b32107bdcc5e901390 ("kernel/cpu.c: create
a CPU_STARTING cpu_chain notifier") added a notify_cpu_starting()
notifier event, and hit every arch except sparc64.

Fix that missed case.

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