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 provided readme file gcc 4.6.2 is supported for some reason. Would this be a feasible expression?
```ruby
fails_with :gcc do
  build "4.6.2"
  cause "libomp can only be built with clang or gcc 4.6.2."
end
```