route: add support for freebsd/arm64
route: add support for freebsd/arm64

This wasn't actually generated on freebsd/arm64 yet. It's mostly a copy
of the amd64 files for now.

Updates golang/go#24715

Change-Id: Iee8c12c29f1d74780948553047bc693668dde0b3
Reviewed-on: https://go-review.googlesource.com/c/156499
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net