x86: use cpumask_var_t in acpi/boot.c

Impact: reduce stack size, use new API.

Replace cpumask_t with cpumask_var_t.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>