kernel: Predicate _cpus_active on CONFIG_PM

This value isn't used outside of the PM subsystem, so don't build it.

More important than the four bytes of .bss was the use of an
atomic_inc().  Some platforms are forced to use
CONFIG_ATOMIC_OPERATIONS_C (but in almost all cases are single-core
devices that won't use atomics at runtime).  There, this turns into a
function call that pulls in the whole atomics implementation.

Signed-off-by: Andy Ross <andyross@google.com>