Incorrect ToInt32 semantics

Hello.

The following unit test fails on armv7-hf linux build and on x86_64 in unittests mode build (unit tests are built with O3):

```
  ecma_number_t m2 = (ecma_number_t) -2.0;
  JERRY_ASSERT (ecma_number_to_int32 (m2) == -2);
```

Seems that `ecma_number_to_int32` is implemented incorrectly.
