add HW_NCPUONLINE sysctl (number of cpus being used) on OpenBSD
add HW_NCPUONLINE sysctl (number of cpus being used) on OpenBSD

rust-lang/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang-nursery/libc