Small improvements to -Xprint-args

  - Don't print `-Xprint-args ...` itself
  - Render a programattically added single output directory as -d.
    Zinc sets the output directory in this way.

```
scala> import scala.tools.nsc._; val g = new Global(new Settings); g.settings.outputDirs.setSingleOutput("/tmp"); g.settings.printArgs.value = "-"; val run = new g.Run(); run.compileSources(g.newSourceFile("") :: Nil)
-d
/tmp
<console>
import scala.tools.nsc._
```

src/compiler/scala/tools/nsc/Global.scala

@@ -1446,7 +1446,24 @@ class Global(var currentSettings: Settings, reporter0: Reporter)
 
     private def printArgs(sources: List[SourceFile]): Unit = {
       if (settings.printArgs.isSetByUser) {
-        val argsFile = (settings.recreateArgs ::: sources.map(_.file.absolute.toString())).mkString("", "\n", "\n")
+        val saved = settings.printArgs.value
+        val argsFile = try {
+          val singleOuputDir: List[String] = if (settings.d.value == settings.d.default) {
+            settings.outputDirs.getSingleOutput match {
+              case Some(file) =>
+                val jfile = file.file
+                if (jfile != null && !java.nio.file.Files.isSameFile(jfile.toPath, java.nio.file.Paths.get(settings.d.value))) {
+                  // A build tool must have used `settings.outDirs.setSingleOutput`, bypassing `-d`.
+                  // Render that to the equivalent -d arguments.
+                  "-d" :: jfile.toString :: Nil
+                } else Nil
+              case _ => Nil
+            }
+          } else Nil
+          (settings.recreateArgs ::: singleOuputDir ::: sources.map(_.file.absolute.toString())).mkString("", "\n", "\n")
+        } finally {
+          settings.printArgs.value = saved
+        }
         settings.printArgs.value match {
           case "-" =>
             reporter.echo(argsFile)