[macOS M1] Backports from upstream to fix cpu.Info() crashes

[KSerrania]

Since #40, we can build projects containing this gopsutil fork on macOS M1.

However, using cpu.Info() causes a crash: sysctl hw.cpufrequency doesn't return anything on M1, which makes the following snippet crash:

out, err = invoke.Command(sysctl, "hw.cpufrequency")
	if err != nil {
		return ret, err
	}

	values := strings.Fields(string(out))
	hz, err := strconv.ParseFloat(values[1], 64)

with an index out-of-range error.

This PR:

• backports Lomanic@2ec3560, which uses golang.org/x/sys/unix.Sysctl instead of manual calls to the sysctl binary, preventing the crash
• backports [cpu][mac] change not return error even if cpu.frequency errors shirou/gopsutil#1192 to not make cpu.Info() return an error when the CPU frequency cannot be found, to prevent the method from erroring on M1.

The drawback of this approach is that the CPU frequency won't be available on M1, but according to the discussions in shirou#1000 and giampaolo/psutil#1892, no definitive solution to this issue has been found yet.