unix: regenerate sysnum files for freebsd/arm64 and netbsd/arm64

The zsysnum_{freebsd,netbsd}_arm64.go files were generated before
mksysnum was converted to Go in CL 152677. Regenerate them to adjust the
comment specifying the command used for generation.

Change-Id: I8f00249848fe20e388f255e9e615da1e0727c140
Reviewed-on: https://go-review.googlesource.com/c/159778
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zsysnum_freebsd_arm64.go

-// mksysnum_freebsd.pl
+// go run mksysnum.go https://svn.freebsd.org/base/stable/10/sys/kern/syscalls.master
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 // +build arm64,freebsd

unix/zsysnum_netbsd_arm64.go

-// mksysnum_netbsd.pl
+// go run mksysnum.go http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/sys/kern/syscalls.master
 // Code generated by the command above; DO NOT EDIT.
 
 // +build arm64,netbsd