Improve breakpoint handling

This fixes #2519, by making sure the disasm stuff runs on its thread, not on the CPU thread.  Seems better that way even if it didn't fix anything.

Also allows hitting go when the pc is on a breakpoint, and actually going as you'd expect.  It doesn't work for branches or delay slots, though.  Maybe a vector would work, but I think it's checking for the breakpoint too much in those cases... anyway, this improves the common case in a definite way.

-[Unknown]