syscall: openbsd amd64 syscall support

Add support for syscalls on openbsd amd64. This is based on the
existing freebsd amd64 implementation.

R=mikioh.mikioh, rsc, yourcomputerpal
CC=golang-dev
https://golang.org/cl/4798060

src/pkg/syscall/syscall_openbsd_amd64.go

@@ -0,0 +1,42 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syscall
+
+func Getpagesize() int { return 4096 }
+
+func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
+
+func NsecToTimespec(nsec int64) (ts Timespec) {
+	ts.Sec = int32(nsec / 1e9)
+	ts.Nsec = nsec % 1e9
+	return
+}
+
+func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
+
+func NsecToTimeval(nsec int64) (tv Timeval) {
+	nsec += 999 // round up to microsecond
+	tv.Usec = nsec % 1e9 / 1e3
+	tv.Sec = int64(nsec / 1e9)
+	return
+}
+
+func SetKevent(k *Kevent_t, fd, mode, flags int) {
+	k.Ident = uint32(fd)
+	k.Filter = int16(mode)
+	k.Flags = uint16(flags)
+}
+
+func (iov *Iovec) SetLen(length int) {
+	iov.Len = uint64(length)
+}
+
+func (msghdr *Msghdr) SetControllen(length int) {
+	msghdr.Controllen = uint32(length)
+}
+
+func (cmsg *Cmsghdr) SetLen(length int) {
+	cmsg.Len = uint32(length)
+}