sys/cpu/x86_64: Expose CPUMASK macros to userspace without _KERNEL_STRUCTURES

sys/cpu/x86_64: Expose CPUMASK macros to userspace without _KERNEL_STRUCTURES

Userspace programs other than /sbin/usched may use cpu affinity,
as the syscall was added for userspace programs to control it,
so it should not require _KERNEL_STRUCTURES.

Also note that cpumask_t which is a structure used by CPUMASK
macros doesn't require _KERNEL_STRUCTURES.

Confirmed the change doesn't break buildworld and buildkernel/LINT64.

(I actually had compile-time issues with fio while trying to add
cpu affinity support, and ended up copy-pasting CPUMASK macros
to a DragonFly specific header in fio source without defining
_KERNEL_STRUCTURES)