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_darwin.go

@@ -394,6 +394,18 @@ func Uname(uname *Utsname) error {
 	return nil
 }
 
+func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
+	if raceenabled {
+		raceReleaseMerge(unsafe.Pointer(&ioSync))
+	}
+	var length = int64(count)
+	err = sendfile(infd, outfd, *offset, &length, nil, 0)
+	written = int(length)
+	return
+}
+
+//sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)
+
 /*
  * Exposed directly
  */