x/sys/unix: add sysctl system call on Linux

[kusattapansan10]

I noticed that golang.org/x/sys/unix don't have the sysctl function. Most compiler language like c have it by default.
So I think go should have sysctl

[seankhliao]

Linux has deprecated / removed sysctl https://man7.org/linux/man-pages/man2/sysctl.2.html
Why should we add it?

[kusattapansan10]

I don't think it's not a system call
Can be read from /proc/sys directory

It's still required to doing some things in linux.
For example to see cpu temperature in linux reading a sysctl value "hw.acpi.thermal.tz0.temperature" is most common way as far as I know

[ianlancetaylor]

It's not necessary to open a proposal to add a simple system call wrapper to x/sys/unix, so taking this out of the proposal process.