[wpiutil] Add JNI access to C++ stderr (#4381)

This is useful in some debugging scenarios.  System.err is separately buffered, so when e.g. debugging test cases it doesn't interleave correctly with the C++ stdout/stderr logging. Even using flush() doesn't seem to help, I think because Gradle does its own buffering.

wpiutil/src/main/java/edu/wpi/first/util/WPIUtilJNI.java

@@ -54,6 +54,8 @@ public class WPIUtilJNI {
     libraryLoaded = true;
   }
 
+  public static native void writeStderr(String str);
+
   public static native void enableMockTime();
 
   public static native void disableMockTime();