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>