unix: use vm.swap_total for FreeBSD TestSysctlUint64
unix: use vm.swap_total for FreeBSD TestSysctlUint64

Fixes golang/go#21375

Change-Id: Ibb33df9d71eb3a935a1f8f9f0ad1f180211e8ce4
Reviewed-on: https://go-review.googlesource.com/54390
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys