Arm: cpu_*_map adjustments

Arm: cpu_*_map adjustments

First, they all start out zeroed. There's no point doing an initial
cpumask_clear() on them.

Next, only cpu_online_map may be altered post-boot, and even that only
rarely. Add respective placement attributes.

Finally, cpu_present_map really isn't anything more than an alias of
cpu_possible_map. Avoid the copying, and have the linker provide the
symbol (if needed in the first place; it is needed right now as
common code references the symbol).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>