ios patch no 8: adjustor pools

"Adjustor" is the term used for a C function pointer that allows C code to call back to Haskell. Normally these are generated at runtime.

However, the iOS kernel doesn't allow self-modifying code. So, on iOS we use a pool of precompiled adjustors of a fixed size, and this patch is the implementation for that.

It consists of three parts:

1. A POOLSIZE pragma, that is used like this:

  foreign import ccall safe "wrapper" {-# POOLSIZE 100 #-}
    mkDelegate :: IO () -> IO (FunPtr (IO ()))

This patch makes this pragma work on all platforms, but it'll have no effect on platforms other than iOS.

I am not sure what the procedure is for additions of pragmas. Do pragmas require {-# LANGUAGE xx #-} ? Anyway, please review whether the approach taken here is acceptable.

1. The Haskell code in the compiler to generate the stubs for the pooled adjustors.
2. The runtime system's implementation of pooled adjustors in C.