sys: Extract CPUMASK macros to new <machine/cpumask.h>

sys: Extract CPUMASK macros to new <machine/cpumask.h>

There are plenty enough CPUMASK macros already for them to have their own header.
So far only userspace users are powerd(8), usched(8) and kern_usched.c(VKERNEL64).
After recent change to expose kernel internal CPUMASK macros those got available
for userland codes even through <time.h> header. It is better to avoid that.
Also this reduces POSIX namespace pollution and keeps cpu/types.h header slim.

For now leave CPUMASK_ELEMENTS (not sure about ASSYM() macro handling the _ prefix)
and cpumask_t typedef (forward decl of struct cpumask would be better in prototypes).