Implement PKHBT and PKHTB on dyncom, fix on armemu

Implements PKHBT and PKHTB on dyncom and also fixes several issues with them on armemu. Previously, armemu was not doing an arithmetic shift right, and was also incorrectly decoding the immediate field. Ran against the valgrind tests. Here are the outputs:
* armemu new: https://gist.github.com/bunnei/fbeec35d19e02c50c30b
* armemu (old, when it was broken): https://gist.github.com/bunnei/f073e383cf8d6ae147c5
* dyncom: https://gist.github.com/bunnei/3a5e7fe70f28f1e76574
* hardware: https://gist.github.com/bunnei/965618d28d0e7e083a83
* valgrind: https://github.com/liquid-mirror/valgrind/blob/master/none/tests/arm/v6media.stdout.exp (note: not the same as hardware!)