6.3 was EOL'd more than three years ago!

We can't use unix.Sysctl* for some sysctls, so we're on our own with
converting data from C arrays.

Don't assume that the byte order is little endian but do the right
thing.  Moreover, there's a little distinction in the sizes reported
by KERN_CPTIME (long[cpustates]) and KERN_CPTIME2
(u_int64_t[cpustates]) so account for that too.

don't make assumptions on which CPUs are online and wich aren't based
on hw.smt and hw.ncpuonline.  Rather, use KERN_CPUSTATS to get the CPU
statistics, which includes a flag field that can tell us if that CPU
is online or not.

Even thought OpenBSD often breaks the ABI compatibility and doesn't make
*any* promise of "stability", this project aims to be "pure go" so avoid
doing inter-op at the cost of artificially reducing the number of
supported architectures down to amd64 and i386.

To add support for another architecture (e.g. arm), add another file
cpu_openbsd_${arch}.go like done for 386 and amd64.  The fields are
declared as `long' in C, so pick the appropriate size when declaring the
struct.