Improve sync and pause GE signal handling

We don't actually do the continue signal properly.  Technically, the list ought to continue while the CPU interrupt is running.  However, it should not be able to complete (until after the interrupt), and a future GE signal should queue up afaict.

Disabling sync fixes rescheduling not happening right.  It could just be bad gpu cycle timing.  Since it never needs to run any userspace code, this may be fine.  The only downside is, when multithreading it probably is good to actually "sync" with the cpu... but that's a separate problem anyway, it should be doing that "automatically."

Unfortunately, I only have a few games that even use pause:
 * Hexyz Force (#3973): this fixes the pause issue, but reveals a new issue (pixel poking.)  So now the screen is black, but the text is visible.
 * Savage Moon: not sure it fixes anything, but works fine.
 * Kingdom Hearts (fixes #4541): makes it work properly with threading on or off.

Possibly may help #3669 which I do not have to test.

-[Unknown]