Summary:
Fix #9423.

The problem in #9423 is caused when code invoked by `hs_exit()` waits
on all foreign calls to return, but some IO managers are in `safe` foreign
calls and do not return. The previous design signaled to the timer manager
(via its control pipe) that it should "die" and when the timer manager
returned to Haskell-land, the Haskell code in timer manager then signalled
to the IO manager threads that they should return from foreign calls and
`die`. Unfortunately, in the shutdown sequence the timer manager is unable
to return to Haskell-land fast enough and so the code that signals to the
IO manager threads (via their control pipes) is never executed and the IO
manager threads remain out in the foreign calls.

This patch solves this problem by having the RTS signal to all the IO
manager threads (via their control pipes; and in addition to signalling
to the timer manager thread) that they should shutdown (in `ioManagerDie()`
in `rts/Signals.c`. To do this, we arrange for ea...