The regabi builders are unhappy about badctxt calling throw
calling systemstack calling gosave_systemstack_switch calling
badctxt, all nosplit, repeating. This wouldn't actually happen
since after one systemstack we'd end up on the system stack
and the next one wouldn't call gosave_systemstack_switch at all.
The badctxt call itself is in a very unlikely assertion failure
inside gosave_systemstack_switch.
Keep the assertion check but call runtime.abort instead on failure,
breaking the detected (but not real) cycle.