Fix #4988: we were wrongly running exception handlers in the

maskUninterruptible state instead of ordinary mask, due to a
misinterpretation of the way the TSO_INTERRUPTIBLE flag works.

Remarkably this must have been broken for quite some time.  Indeed we
even had a test that demonstrated the wrong behaviour (conc015a) but
presumably I didn't look hard enough at the output to notice that it
was wrong.