unix: remove raw syscall from Sendfile

Update golang/go#17490

Change-Id: Iaec54b8ffda1a24d4c8b5671185d570fb8683155
Reviewed-on: https://go-review.googlesource.com/c/154663
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_openbsd.go

@@ -94,6 +94,13 @@ func Getwd() (string, error) {
 	return string(buf[:n]), nil
 }
 
+func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
+	if raceenabled {
+		raceReleaseMerge(unsafe.Pointer(&ioSync))
+	}
+	return sendfile(outfd, infd, offset, count)
+}
+
 // TODO
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	return -1, ENOSYS