[wpiutil] Add GetSystemTime() (#3840)

This portably gets the time in microseconds since the Unix epoch.

wpiutil/src/main/native/cpp/jni/WPIUtilJNI.cpp

@@ -116,6 +116,18 @@ Java_edu_wpi_first_util_WPIUtilJNI_now
   }
 }
 
+/*
+ * Class:     edu_wpi_first_util_WPIUtilJNI
+ * Method:    getSystemTime
+ * Signature: ()J
+ */
+JNIEXPORT jlong JNICALL
+Java_edu_wpi_first_util_WPIUtilJNI_getSystemTime
+  (JNIEnv*, jclass)
+{
+  return wpi::GetSystemTime();
+}
+
 /*
  * Class:     edu_wpi_first_util_WPIUtilJNI
  * Method:    addPortForwarder