Exception handlers should share a stack

[projectgus]

As mentioned originally by @foogod and came up again in #75, exception handlers use the current running task's stack space.

This means the minimal stack size is around 128 words (512 bytes), even for very simple tasks.

A more resource-friendly option will be to have a single exception handler stack that is switched to whenever exceptions run. The NMI already does this, but the "normal" exception handlers do not.

Additionally, if we get a bit more of the lower layers reverse engineered then it may be possible to change any stack-intensive parts of the handlers to use statically allocated memory instead.