Keep GNU fastpath for jerry_port_get_local_time_zone_adjustment (#4600)

PR #4513 caused a measurable slowdown by removing the GNU specific TZA calculation.
This patch reverts this removal but also keeps the general implementation introduced in #4513

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik robert.fancsik@h-lab.eu

jerry-port/default/default-date.c

@@ -59,7 +59,19 @@ static double FileTimeToUnixTimeMs (FILETIME ft)
 double jerry_port_get_local_time_zone_adjustment (double unix_ms,  /**< ms since unix epoch */
                                                   bool is_utc)  /**< is the time above in UTC? */
 {
-#ifdef _WIN32
+#if defined (HAVE_TM_GMTOFF)
+  struct tm tm;
+  time_t now = (time_t) (unix_ms / 1000);
+  localtime_r (&now, &tm);
+
+  if (!is_utc)
+  {
+    now -= tm.tm_gmtoff;
+    localtime_r (&now, &tm);
+  }
+
+  return ((double) tm.tm_gmtoff) * 1000;
+#elif defined (_WIN32)
   FILETIME fileTime, localFileTime;
   SYSTEMTIME systemTime, localSystemTime;
   ULARGE_INTEGER time, localTime;
@@ -111,11 +123,11 @@ double jerry_port_get_local_time_zone_adjustment (double unix_ms,  /**< ms since
   }
 
   return tza_s * 1000;
-#else /* !_WIN32 && !__GNUC__ && !__clang__ */
+#else /* !HAVE_TM_GMTOFF && !_WIN32 && !__GNUC__ && !__clang__ */
   (void) unix_ms; /* unused */
   (void) is_utc; /* unused */
   return 0.0;
-#endif /* _WIN32 */
+#endif /* HAVE_TM_GMTOFF */
 } /* jerry_port_get_local_time_zone_adjustment */
 
 /**