This adds an -openmp option (off by default) for building the OpenMP run-time.
The run-time is still not well integrated with the regular LLVM build, but this allows building the run-time "on the side".
I'm thinking that we could provide separate pre-builts of the run-time (e.g. for x86_64 and Mac) at http://llvm.org/releases/download.html as a convenience for users who wish to experiment with Clang's OpenMP run-time.
Let me know what you think.