internal/syscall/unix: consolidate kernelVersion implementations

Currently, there are 3 functions returning Linux kernel version numbers.

Two of them are identical:
 - in net, initially added by commit 0a9dd47dd817904e;
 - in internal/poll, initially added by commit 1c7650aa93bd53;

(both were later fixed by commit 66c02645062561a).

The third one is a more complex, regexp-based implementation in
runtime/pprof, which is only used for a test.

Instead of adding one more, let's consolidate existing ones.

Remove the complex implementation, and move the simple one into
internal/syscall/unix. Use it from all the three places mentioned above.

Change-Id: I4a34d9ca47257743c16def30e4dd634e36056091
Reviewed-on: https://go-review.googlesource.com/c/go/+/424896
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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