modifications to RTS to support 8-bit platforms

There are three primary changes:

 1. add HAS_PTHREADS so that we can build on platforms that do not support threading
 2. give some variables more precise types so that they still have the correct number of bits on 8-bit platforms
 3. add a minor hack that forces memory allocated in the heap to build 16-bit aligned.

There is also some minor stylistic cleanup. All the changes should be fairly well isolated into separate patches.

These changes, plus a few minor hacks, made it possibly to run Idris on an Arduino. I will submit more patches when I have turned the hacks into acceptable solutions.