Process starttime non-cgo implementation

From https://github.com/elastic/go-sysinfo/issues/127#issuecomment-1400401778

> I am using the startup time for the Java agent attacher
> However, it is not the most crucial piece of info I rely on. Even if I don't do anything, processes will be only identified only based on their PID. That should be good enough as the cache I am maintaining gets updated every second, including the removal of stale process entries, which means there will be a problem only if a process is terminated and another starts with the same PID within one second, which is unlikely enough for the macOS use case (non-production).

APM Server is built without CGO but we need the process starttime for the java attacher and that requires CGO.

Solution: implement process starttime without CGO