Pass the correct probed xxx_HOST_OS and xxx_HOST_ARCH options to hsc2hs

These values are taken from the target platform returned by an invocation of ghc --info.
The method that Configure.hs used was correct. Now the correct function is used by both PreProcessor.hs and Configure.hs.
Previously PreProcessor.hs assumed that host == build, which meant it didn't work right for cross compiling (iOS in my case).