Build CPUInfo on Darwin

[localhots]

cpuinfo.go is currently only built on Linux and it breaks Darwin builds for no apparent reason. I understand that there's no procfs on Darwin but it should still be possible to use macOS for development of projects that use this package.

This change allows it to be built on Darwin with a dummy parseCPUInfo implementation.

Please let me know if I need to change this solution in any way. Thanks!