HAL_SITL: add methods to fetch native system clock info

libraries/AP_HAL_SITL/system.cpp

@@ -177,4 +177,45 @@ uint64_t millis64()
     return ret;
 }
 
+
+uint32_t native_micros()
+{
+    return native_micros64() & 0xFFFFFFFF;
+}
+
+uint32_t native_millis()
+{
+    return native_millis64() & 0xFFFFFFFF;
+}
+
+/*
+  we define a millis16() here to avoid an issue with sitl builds in cygwin
+ */
+uint16_t native_millis16()
+{
+    return native_millis64() & 0xFFFF;
+}
+    
+
+uint64_t native_micros64()
+{
+    struct timeval tp;
+    gettimeofday(&tp, nullptr);
+    uint64_t ret = 1.0e6 * ((tp.tv_sec + (tp.tv_usec * 1.0e-6)) -
+                            (state.start_time.tv_sec +
+                             (state.start_time.tv_usec * 1.0e-6)));
+    return ret;
+}
+
+uint64_t native_millis64()
+{
+    struct timeval tp;
+    gettimeofday(&tp, nullptr);
+    uint64_t ret = 1.0e3*((tp.tv_sec + (tp.tv_usec*1.0e-6)) -
+                          (state.start_time.tv_sec +
+                           (state.start_time.tv_usec*1.0e-6)));
+    return ret;
+}
+
+
 } // namespace AP_HAL