os: split wait6 syscall wrapper into per-platform files

os: split wait6 syscall wrapper into per-platform files

There are getting to be enough special cases in this wrapper that
the increase in clarity from having a single file is starting to be
outweighed by the complexity from chained conditionals.

Updates #50138.
Updates #13987.

Change-Id: If4f1be19c0344e249aa6092507c28363ca6c8438
Reviewed-on: https://go-review.googlesource.com/c/go/+/442575
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>