Port mksysnum.pl Perl script to mksysnum.go.
mkall scripts are modified to run mksysnum.go.
Running ./mkall.sh does not generate any git diff besides
the command name in comments of generated files.

Updates golang/go#27779

Change-Id: Id62df9d819f00729ab19b6bc4d32dddcf8d9832b
Reviewed-on: https://go-review.googlesource.com/c/151377
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>