syscall: drop compatibility for FreeBSD < 10.0

syscall: drop compatibility for FreeBSD < 10.0

Change-Id: Idd8cee84215e61817a86915160c91242670798af
Reviewed-on: https://go-review.googlesource.com/c/go/+/431663
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Dmitri Goutnik <dgoutnik@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>