Add initial rough support for synthesizing linker options when passed
-fopenmp in the link step on Linux. There is probably more tweaking that
will need to take place to get good support for linking the relevant
libraries on all Linux distributions and/or on other platforms, but this
get's the ball moving and allows Clang to build programs which contain
OpenMP pragmas that can be safely ignored by a compiler that doesn't
implement them, and yet makes direct calls into the OpenMP runtime.