[flang] Generate main only when a Fortran program statement is present (#89938)

This patch changes the behaviour for flang to only create and link to a
`main` entry point when the Fortran code has a program statement in it.

This means that flang-new can be used to link even when the program is
a mixed C/Fortran code with `main` present in C and no entry point
present in Fortran.

This also removes the `-fno-fortran-main` flag as this no longer has any
functionality.