ISACaller: fix syscall emulation

ISACaller: fix syscall emulation

there were two bugs fixed:
1. sc emulation was missing a `return`, so it tried to run sc
   again after running sc and rfid, giving the wrong CIA and
   MSR values.
2. the code to replace and restore the instruction with rfid
   had the wrong endian on the load, so it was corrupting the
   instruction for the next time it was used. I just deleted
   the save/replace/restore code since it isn't needed anymore.

I then changed the syscall tests to ensure both the
bugs above don't happen again.