GHC: Build normal and profiling libraries in parallel.

This looks quite straightforward.

Most probably we have to add a little bit of code to terminate early if there is an error in any of the threads, but that shouldn't be too hard. What are your thoughts on this?

Also, do you see any other problematic points?

The speed-up gained by this is significant (factor 2 for all my code, since I always have profiling enabled).