disable non-emcc runs in test_uncaught_exception

tests/runner.py

@@ -1403,6 +1403,8 @@ if 'benchmark' not in str(sys.argv) and 'sanity' not in str(sys.argv):
         self.do_run(src, 'Throw...Construct...Catched...Destruct...Throw...Construct...Copy...Catched...Destruct...Destruct...')
 
     def test_uncaught_exception(self):
+        if self.emcc_args is None: return self.skip('no libcxx inclusion without emcc')
+
         Settings.EXCEPTION_DEBUG = 0  # Messes up expected output.
         Settings.DISABLE_EXCEPTION_CATCHING = 0