Changing GC mark phase to be non-recursive. It is noticeably slower on some test cases, but doesn't cause stack overflow.

tests/jerry/gc.js

@@ -12,6 +12,15 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+var obj = {}, obj_l;
+obj_l = obj;
+
+for (var k = 0; k < 1500; k++)
+{
+  obj_l.prop = {};
+  obj_l = obj_l.prop;
+}
+
 function f (o, i) {
   if (--i > 0) {
     f ({a:o, b:o}, i);