[flang] Install Fortran_main library

At the moment the Fortran_main library is not installed, so it cannot be
found by the driver when run from an install directory. This patch fixes
the issue by replacing llvm_add_library with add_flang_library, which
already contains all the proper incantations for installing a library.
It also enhances add_flang_library to support a STATIC arg which forces
the library to be static even when BUILD_SHARED_LIBS is on.

Differential Revision: https://reviews.llvm.org/D124759



Co-authored-by: Dan Palermo <Dan.Palermo@amd.com>