cpumask: introduce assign_cpu() macro

Now that assign_bit() is a thin macro wrapper around set_bit() and
clear_bit(), we can use it in cpumask API and drop duplicating
implementations of set_cpu_xxx() helpers with no additional overhead.

Bloat-o-meter reports almost 2k less of generated code for allyesconfig,
mostly in kernel/cpu.c:
	add/remove: 2/4 grow/shrink: 3/4 up/down: 498/-2228 (-1730)

Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>