unix: update openbsd/amd64 to OpenBSD 6.3

Change-Id: I0a16c77437e4ed68cc19eda41bb9b64ab4376fd0
Reviewed-on: https://go-review.googlesource.com/114075
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_openbsd_amd64.go

@@ -31,3 +31,7 @@ func (msghdr *Msghdr) SetControllen(length int) {
 func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
+
+// SYS___SYSCTL is used by syscall_bsd.go for all BSDs, but in modern versions
+// of openbsd/amd64 the syscall is called sysctl instead of __sysctl.
+const SYS___SYSCTL = SYS_SYSCTL