Added libomp formula for 64 bit OS X.

libomp is the OpenMP Runtime Library by Intel. libomp dylib and headers are needed in Xcode projects to write OpenMP programs using clang.
according to the Readme file there are 3 archs supported:
* 32: IA-32
* 32e: x86_64
* mic: many integrated core architecture

As there is nothing about x86, I assume its not working on 32 bit Systems. But I didn't find anything more specific :(