Incorrect implementation of `get_available_pages()` for SemiSpace

[k-sareen]

I think the SemiSpace get_available_pages() [1] should be dividing the heap into two like in the JikesRVM mmtk [2]. Though it seems like get_available_pages() is only used by the generational plans so our SemiSpace implementation is technically not incorrect. Regardless we should fix it.

[1]: https://github.com/mmtk/mmtk-core/blob/master/src/plan/global.rs#L268-L281 [2]: https://github.com/JikesRVM/JikesRVM/blob/master/MMTk/src/org/mmtk/plan/semispace/SS.java#L169-L172