Remove stack alignment hacks.

Now that the problem is fixed on Haiku side, we can remove this
workaround.
Enable DFG JIT.

This is an extra level of JIT that is used for functions that are called
often or loop a lot. It is an optimizing JIT, trying to remove dead
code, inline function calls, and do other stuff to inclrease
performance. This should boost our JS benchmark scores a bit more.