Rather doing one initialization pass over all the per-cpu
cpu_sibling_maps at boot, update the maps at cpu online/offline time.

This is a behavior change -- the thread_siblings attribute now
reflects only online siblings, whereas it would display offline
siblings before.  The new behavior matches that of x86, and is
arguably more useful.
Signed-off-by: NNathan Lynch <ntl@pobox.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>