Also print number of available CPUs on Linux

[arichardson]

Summary

Without this change the buildworld/buildkernel epilogue looks like this:

World built in 249 seconds, sysctl: cannot stat /proc/sys/hw/ncpu: No such file or directory

ncpu: , make -j72.

[mjg]

Side note but I find the current message to be quite lackluster as it does not include user nor system time.

Is there an easy way to include time-like output instead? If time-prefixed execution is a problem then as a hack this perhaps can run "procstat rusage" on itself before and after.

[emaste]

OK with me