pstq emulation correctness #481

In little-endian mode, stq stores the even (lower) numbered register at addr+8 and the odd register at addr.

pstq changed to store the even register at addr and odd at addr+8 regardless of endian.

It looks like load/store emulation may be treating pstq the same as stq here.

Should add quadword instructions to alignment_handler.c selftests, and then fix pstq (and possibly plq etc) if they show up problems.