Implement faster System.Timeout.timeout for the threaded RTS.

The basic idea is to use the Timer Manager rather than spawning an
auxilliary thread to handle timeouts; this was proposed by Bas van Dijk
two years ago (#4963), and recently by Herbert Valerio. The key idea
to make this work is to spawn an auxilliary thread for the delivery
of the Timeout exception. This idea as well as most of the code is due
to Akio Takano.
Build fix for iOS; fixes #7759

Patch from Stephen Blackheath.

The issue here is that the #defines EVFILT_READ and EVFILT_WRITE have
the values -1 and -2. The original code translates that to
filterRead = Filter -1 which is wrong Haskell and fails to compile.
The modified code produces the correct code filterRead = Filter (-1)
Fix warnings