Set number of cpus to 1 if `os.cpus().length` is undefined.

I was working on trying to build cjdns on my iPhone and I came across a problem. `Os.cpus().length` always returned `undefined` for me. I'm not sure the percentage of systems that do have this problem, but I'm sure there are some if I've found one. 

Anyways, if node can't figure out how many cpus a user has it should really just default to 1 so there are no problems.