Modify unix/mksyscall.go to support plan9.
This does not generate any git diff besides the
command name in comments of generated files and
addition of +build tags.

Updates golang/go#27779

Change-Id: I359a80d1c08837d92a7095e5235867c29a2d2a60
Reviewed-on: https://go-review.googlesource.com/c/156157
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>