unix: replace "mksyscall.pl" script with a Go program

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/150217
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/zsyscall_openbsd_arm.go b/unix/zsyscall_openbsd_arm.go
index 5991165..fa1beda 100644
--- a/unix/zsyscall_openbsd_arm.go
+++ b/unix/zsyscall_openbsd_arm.go
@@ -1,4 +1,4 @@
-// mksyscall.pl -l32 -openbsd -arm -tags openbsd,arm syscall_bsd.go syscall_openbsd.go syscall_openbsd_arm.go
+// go run mksyscall.go -l32 -openbsd -arm -tags openbsd,arm syscall_bsd.go syscall_openbsd.go syscall_openbsd_arm.go
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 // +build openbsd,arm