Tests for 0xDEADBEEF register filling

It seems clear that a1, a0-a4, and t0-t9 are filled with 0xDEADBEEF after a syscall.  I don't like this at all.  Apparently games can depend upon it (e.g. Tales of VS.)

Doesn't seem to happen for interrupts, though, so it doesn't look like the thread scheduler's job.

I did not specifically test v1, but since that is for return values I'm not expecting it to be overwritten.

Implementing this in PPSSPP is gonna suck for debugging.

-[Unknown]