cpumask: x86: convert cpu_sibling_map/cpu_core_map to cpumask_var_t

Impact: reduce per-cpu size for CONFIG_CPUMASK_OFFSTACK=y

In most places it's cleaner to use the accessors cpu_sibling_mask()
and cpu_core_mask() wrappers which already exist.

I couldn't avoid cleaning up the access in oprofile, either.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>