unix/linux: replace "mksysnum.pl" script with a Go program

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>
diff --git a/unix/zsysnum_linux_386.go b/unix/zsysnum_linux_386.go
index 8f33ece..8d17873 100644
--- a/unix/zsysnum_linux_386.go
+++ b/unix/zsysnum_linux_386.go
@@ -1,4 +1,4 @@
-// linux/mksysnum.pl -Wall -Werror -static -I/tmp/include -m32 /tmp/include/asm/unistd.h
+// go run linux/mksysnum.go -Wall -Werror -static -I/tmp/include -m32 /tmp/include/asm/unistd.h
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 // +build 386,linux