[flang] Add a wrapper for Fortran main program

Add a C wrapper that calls the Fortran runtime initialization and
finalization routines as well as the compiled Fortran main program
_QQmain.

Place it in its own library to satisfy shared library builds since it
contains a C main function.

- https://github.com/flang-compiler/f18-llvm-project/commit/cc7ac498f916d32a9b197d3ace816f4de5d36aad#diff-fa35a5efa62731fd2845e5e982eca9a2e36439783e11a4e4a463753c2160ec10R53
- was created in flang/test/Examples/main.c in Eric's branch