Port mksyscall.pl Perl script to mksyscall.go.
mkall scripts are modified to run mksyscall.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: I8c6eb852e6821f4a91dc03cc042d8d343f1bcf66
Reviewed-on: https://go-review.googlesource.com/c/150217Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>